What can I do?!?

Before I met my SO I was never able to cum during sex, I was only able to by myself. When my SO and I got to together he was the first guy I was ever really comfortable enough with to be able to let go fully and make me cum. Lately (about the past two months with the exception of a few times) when we’re getting frisky and he’s pleasuring me I haven’t been able to focus on myself like before and cum, and because it’s been taking me so long he gets tired, which I understand and have no issues with, so I end up finishing myself. The problem with all that is I know it’s giving him some doubts and he’s feeling like he’s doing something wrong and he’s not, this is 100% all on me. I know he’s having these feeling because his ex used to make him feel that way and then he found out she was cheating on him (which to clarify I AM NOT). I’ve tired not to focus on the fact that it’s been taking me a long time but I still do and then that causes me to loose focus again which in turn makes me take longer. I need some advice on what I can do to change things back to the way they were and not have my SO have any more doubts. TIA