He sounds exhausting. I expect that the reason why you are not able to ignore it is because he has worn you down over many years with his incessant drip-drip-drip of negativity.
Seriously, I think you have to change your response to him. I know I trot this phrase out a lot (found it here and it struck a chord) but "If you do what you've always done, you'll get what you've always got". He is bypassing your usual response and continuing his pissy behaviour. He knows how to respond to your regular responses and to keep wearing you down.
The possibility I see here is that, for example, when he says he 'never' has clean socks - you stop washing his socks. Because to do otherwise would make a liar out of him. 'Never' gets dinner? Stop cooking for him. Show him what 'never' feels like. And tell him that's why you're doing it, and he needs to pick his words better in future.
"Same for holidays/ any kind of trip. If he didn't really want to go on it or whatever, but then decided to do it, I never ever hear the end of it- for the entire trip. He just ruins it."
Don't go on holiday / trips with him. Blank refusal. Tell him why, that he ruins it with his negativity. He sneers at you as a "cop out" if you accept his choice and moans constantly if he accepts your choice so there's no point and you're not bloody well doing that again.
His negativity has a purpose, it is not random. It is designed to train you to accept all and every decision or choice he makes. Not only does he get his own way, but also the pleasure of calling you a cop-out for not putting forward your own choice. It's really shitty behaviour, this. Put forward your own choice, he'll moan and make you miserable. Accept his choice, he sneers at you. He really has designed a heads-I-win-tails-you-lose life for himself, hasn't he? And he likes to make you miserable.
"Apparently I take it too seriously when he does it and I'm too sensitive to it. He says I should stop caring about what he thinks and it's just his personality when he's in a mood."
I see another opportunity to not do what you've always done here. It will take a little acting, but do your best to behave as if his moods do not affect you. I suspect he enjoys being moody, because it's a chance to make you miserable. There's a technique called 'Grey Rock' which you could put to use, you'll find details online, but essentially you do not react to anything. He is being an arse to make you react, so not reacting provides no entertainment for him. He will probably up the provocation to sting you into reacting, but you maintain - Grey Rock.
Of course Grey Rock is more of a holding pattern than a way to live. I think it would be a good idea to consider how you want your life to be. And whether you want this deeply unpleasant man to be in it.