It’s been a while since I’ve been fucked, as whilst I’m bi, I’ve been in a straight relationship and she has no interest in pegging.
My advice is to potentially practice with toys. I’ve got a few of varying sizes, from small, to bigger than any human cock (it’s thick and I can take about 7-8 inches of it). Build up slow, and eventually you will get to the point where you can take it hard and fast and find it enjoyable.
When I’ve actually been fucked, once it’s in and I’m relaxed, it’s usually uncomfortable for about a minute, so I get them to take it easy, and then I can take it as hard as they like.
Also, lube.