I could have written this. Just from the opposite side as you. I agree completely to everything you said. Especially that part about it feeling awful if the other person is just having sex as a favor to you. Long ago I gave up begging and manoeuvring her into sex. I felt lime a rapist pervert, and the sex was not very satisfying. I also elected to just wait until she was ready. Sex was still poor, but better than none. Unfortunately the intervals gradually stretched out. Weeks, then months, then years, then decades. I don't think there is anything that you can do about him, unless you can get him to medical and psychological help. And maybe not then. My wife refused to get help. As she put it, why should she seek help to become a sex crazed nut like me.
It is no wonder that he is a two stroke wonder. His equipment gets no work, is super sensitive, and fires almost immediately before you get the deep pounding you crave. If he used it more, he would last longer. But he doesn't want it to last longer, he wants to get it over with. Unfortunately, he isn't thinking of you.
You are not totally unique. A friend of mine was very much like you. He could be getting laid once every few weeks and was happy. She was thinking that twice a day would maybe get her by. I wish I had been able to help, we both would have felt better. Anyway, she delivered an ultimatum. If he could not screw her every morning, she would find someone who could. He tried. He said that there was no reason that he could not put it to her every morning. But after a couple of weeks, he just could not get it up any more. There is no deep penetration with a limp penis. So, she found a guy who would screw her frequently and well. At least for a while. The couple divorced.
So, that brings us to what you can do.
First, is just accept it. My choice. But I don't recommend it. It leads to a life of frustration and bitterness. Love will not survive if you are never getting any help having sex. Second, find a fuck buddy.
Someone who will do what you want, and then go away. I would have loved this option, but am fairly lousy at seduction and could never find anyone, and was to gutless to try on my friends wife. Missed opportunity. But for a woman, it is fairly easy to find a guy to screw her. A lot of guys are very interested.
And third, go your separate ways now while you still think fondly of each other. Don't wait until you have wasted your life and hate each other out of misery and frustration. This would be my recommendation. You may not find anyone better, but then you might. It would be worth the risk.