Why can’t I finish during sex?

My partner and I have been together for more than 2 years now and every time he and I have sex, he is able to cum (it’s not like it’s quick or anything) but I have never been able to finish. There have been times where it felt like I was going to pee so we had to stop, but now that feeling is gone and I just can’t cum.

I can only make myself finish when I’m masturbating with my clit...