Being on top during sex but so exhausted.

I'm 20 and had heart surgery last year. I still have a multitude of heart problems, take 12 pills a day, and have at least 3 doctors appointments a month. I go 10 hours away to the Cleveland clinic to have tests done and junk. It's awful. Anyway, back when I was 17-18, I used to have sex with my ex all the time, mostly with me on top because it was kinda my thing and I was great at it. 🙈 Now I'm married to a new guy, who is amazing! He clearly knows about my heart and knew before we started dating. However, he loves it when I'm on top. He likes when I grind on his d**k but that hurts so bad (titled uterus makes sex painful and of course cervix probs) and everything I do that I used to be awesome at and make my ex beg for, completely wears me out. I didn't have those problems back then, though. So how can I not be so exhausted while on top? Help?! Want to show my husband what I'm capable of...