Not flexible for sex

I am probably the least flexible person on the planet. It was never something that bothered me until my boyfriend and I started having sex. There are many positions that I can’t be in very long because it hurts and it’s frustrating for the both of us. (He never shames me or anything! It just is a bummer for the both of us having to switch it up so often lol)

It’s the most difficult when I’m on top because he’s a smaller guy overall, a little shorter than me and skinnier than me. I can’t spread wide enough to get low enough if that makes sense.

I obviously know the simple answer is the stretch more, but are there any specific stretches that will help my flexibility for sex?

Thanks!!