I feel horrible for being an old nag to my parent, and I've become impatient at times. I'm feeling so guilty. Advice?

Your profile says dementia, so try to understand repeating yourself isn't going to help. You need to cut back on it. He won't retain what you've said. Rather than directing him to do things, which he may balk at, stick to the most important things that he can manage, and when possible, make offerings/suggestions rather than directives. For instance, instead of harping on him to drink and the repercussions for not drinking, just provide a beverage, preferably something he likes and/or offer a choice of beverage. If he doesn't choose one, just put something out for him and leave it.

There are several choices for the AC situation, but telling him not to touch it isn't one of them! JoAnn29 offered one possible solution. Search for "thermostat cover lock" shows several for under $20. This is the cheapest and easiest solution. Another solution is to replace the current one with something like the Nest. My mother had a regular one, which had several settings/times, but a manual heat/cool switch. I stopped by once and augh, while I don't care for very cold AC, her place was like a sauna and she didn't even notice it! I flipped the switch to AC and it took time to get the place more comfortable! YB replaced it with the Nest. He could monitor and make changes remotely via WiFi. It's possible he's turning the AC off because he's cold. The residents in mom's MC were ALWAYS cold, whether it was winter or summer! Once you can lock the thermostat, compromise on the setting - perhaps a bit warmer than you like, but you can adjust YOUR clothing.

If he's eating some, that's a plus. Perhaps adjusting his meals so they are a little "sweeter", to entice him to eat more might help. Many elders do like their sweets, but it'd be nice to get a little more healthy food in first! If you can work it out, great, otherwise, let him eat what little he will and provide his "sweets."

If you insist on discussing news or politics, he has every right to his "opinion", no? Smart aleck comment or not, it's his right. If you don't like what he thinks or has to say, then don't bring up the topics!

"..he never tells me he loves me back when I tell him I love him."

This is your expectation. It's best to just keep telling him you love him, give him hugs or a hand squeeze and let it go.

"I feel horrible as I tend to nag too much with many worries, constantly reminding him to be careful of almost every situation."

As mentioned above, you are repeating all this and it's going in one ear, out the other. Why not just stop? You are losing patience because he doesn't seem to comply. It isn't him trying to be difficult, it's just how it is. The only time you need to step in is if you think he's in some danger and it should be to gently guide him away from whatever it is, then redirect his attention elsewhere. It'll take practice, but can be done.

It has nothing to do with you being good enough or a control freak or anything like that. He can't retain what you're repeating to him and in many cases the staring, sulking, etc is really just his inability to process what you are saying or asking for.

Practice greeting him, not repeating directions. That time is past. Smile. Touch. Ask if he needs/wants anything. Keep statements simple and bland. Sure, it's boring, but he really can't be/do what you want. He lives in a different reality, so you have to meet him there.

It's not babying him, but you're expecting more than he can manage. Get the thermostat lock. Practice reticence when you feel the need to tell him what to do, not to do, what the consequences will be, etc. It's like dealing with a toddler, who hasn't learned yet and who's mind wanders easily. Don't treat him like a baby, but don't treat him like a fully functioning adult. His mind really can't handle that.

If you slip into reminder modes at times, don't feel guilty. It'll take time/practice to get into the new mode! Keep conversations minimal, ask about the past, let him "drive" the train, so to speak.

The problem with ac is he is cold and you are hot. Get him in to warmer things to wear - thicker tshirts, long sleeve. Have blankets or throws near his chair. All my siblings keep their ac as low as it goes. It is painful to sit in their house. The weirdest part to me is they get it polar bear cold and cover up with blankets. I am the caregiver for parent and, thankfully, we both tend to be cold - so I can deal with keeping it a little warmer inside. I turn it lower when I know others are coming or as soon as I see them drive up. I also bought vent covers that redirect the cold air away from her. In the winter I turn it around so the hot blows at her. They are clear plastic and have magnets that attach to the vent. Very easy.

The no response when you say you love him? That doesn't come natural to everyone if they haven't done it all their life. Next time things are going well, just talk to him. Ask him if he loves you. No expect is as a reply - just ask him. You may both be able to express love to each other during the conversation. You have been a great father...I always loved it when you did XX...etc.

It's a relief to know that I'm not the only caregiver that is going through this. I use to fuss at my mother about her diet but not any more. She will soon be 87 and she got this far without eating healthy why start now? She would rather take a pill for her cholesterol than eat veggies. It is what it is! Her home is stifling hot but when I visit she turns on the air. She doesn't come to my house because it's not her nest so I just keep her comfy and move on.

He's elderly, has dementia, and isn't going to remember what you tell him. As many have answered, the elderly get cold very easily. I'd recommend not setting the temperature low if you put a lock on the control. As far as him eating sweets, let him.

It's all difficult... I know it all so well... We just have to try our best...I've been so guilt ridden throughout the years with all this tug and pull... but, now our parent(s) are extremely frail and old... It's so good you're on here (and me) so we can try and get advise and vent at the same time. Sending you and your family lots of Love.

The day comes when you have to admit that this is not the parent you knew. This is an aging person, losing their memory and all sense of logic.
He can't say I love you back but hold the memories of the times he did. He would wish that he still could if his brain still worked.
It is weird to be in the role of care giver to someone who raised you. It isnt like a child and parent in reverse. It is hard work emotionally and physically.
Your nagging is your frustration. Find an outlet and time for you. It isnt good for your health. It will bring you to a bitter attitude. Help yourself from that.
Your nagging will not make him do what you want. Prioritize and pick your battles. See what can be done differently to save you from asking him to do/not do something. Try a bit of juice in with the water so it tastes better. If he wants to eat only pie, let him. ( or dont keep pie in the house)
Best wishes to you. Recognize this is hard and some days are sad, every day is a new day to try again.

I wanted to clarify some issues and explain a bit further. My Dad was diagnosed with Dementia; however, he has been stable and seems to remember many things. And has not progressed. I sometimes wonder if he has been misdiagnosed. He was dealing with some stress due to my Mom's health when the doctors diagnosed him. He was always lazy and not domesticated. My Mom did everything for him. I am not using this to excuse why I tend to nag or become frustrated, impatient.  He was always very cheap and stingy and frequently gave you a look, and it always made you feel stupid or uncomfortable. He always had a smart comment when he was younger, and it would be direct, with no tone behind it. He is an extreme introvert. He never tells me he loves me back when I tell him I love him. He did the same thing to my Mom when she told him she loved him. Even before he was diagnosed. And yes, I feel horrible as I tend to nag too much with many worries. I feel like I'm not good enough because I lose my patience by repeating and answering with a frustrated tone of voice. I know I can be a perfectionist and a controlling person trying to have everything go right and need to back off. On the other hand, he can be so stubborn staring back at me, gunning me off, or he can sulk at times. He has always been stubborn and an introvert. When he sulks, I feel so bad and apologize; however, he can sometimes play on it. Also, he is a good man. Don't receive me wrong he had excellent advice to give when I needed it. It is the guilt that eats me, and I know it somehow is related to my Mom's Catholic background of her pushing it on me. Although now he is eating a little better as the weather has cooled down, I say eat what you can, and I've changed his diet and menu.