I can't stay hard when I try to top my boyfriend

So... I just entered a new relationship and it's been great. The sex is pretty mind-blowing as well. In my past sexual escapades, I generally top (my first time bottoming was pretty terrible). However, this time around I have been bottoming every time and it's incredible. My new boyfriend is hung AF! I'm also above average. So, a strange thing has been happening. As I'm about to "stick it in" (for loss of better words haha), I start going soft and just can't do the deed. I'm 23, so it's not an age thing, my boyfriend also has the most incredible ass. I don't know why it is happening, I'm pretty good if I don't say so myself. It's super frustrating and embarrassing. My boyfriend is amazing about it but I feel like crap... any idea what could be happening and what I can do to fix it?