His argument is "we've been together for over 2 years, we have sex nearly every time we see each other so what's the problem"
Problem 1 is that the sex you are having is not the stuff you want to have with him and you know he is capable of performance with his other partner.
Problem 2 is that he is avoiding giving you a reason for this. He wants to brush it under the rug.
when I tried to say I am happy he's found someone to explore those things with but it is difficult for me when I've been craving and asking if we can do these things for ages... well then he said I make him feel bad.
Nope. He just feels bad. You can't make him feel anything. If you had this magical power you could make him feel excited to have anal/kink/ whatever sex with you that you are not having.
Am I being neurotic or is there a problem here?
Yes. In communication. You should be able to ask your partner "Would you be willing to share sex with me in this manner?"
And he should be able to communicate back "Yes, I would be willing."
Or " No, I would not be willing."
And you should be able to inquire why if not willing.
And he should be able to give you a reasonable answer.
"I have frogs in my pants." ( I joke on purpose to keep it light. Not trying to minimise or devalue your feelings.)
Then you could ask him if he needs help removing the frogs or if this is just how it is now. Permanent frogs.
So you can then choose to accept him as he is now, and adjust your want to have this kind of sex with him or not.
Since you have this kind of sex with your other partner, I suspect it is not the mere sex acts. It's the intimacy -- emotional intimacy perhaps. Sex is both an expression of physical intimacy. Or a combo thing of (physical intimacy+ emotional intimacy.) If he has aged and now has frogs in his pants, how else do you share emotional intimacy together other than via sex?
Since he's unwilling to TALK about Shared Vulnerable -- and thus help create emotional intimacy with heart-to-heart talks... where else do you guys do this together? Share emotional intimacy? Could these things be plumped up a bit? Does he write you love letters? Sing you songs? What?
Is he of age for andropause? Could that be affecting him?