Sexual Flexibility

i want to be able to ride my ma-...ok he’s not my man but i want to ride his dick into the sunset. I’m a big girl and whenever i like playfully get on top of him in that position my legs feel really tight and uncomfortable. Like riding him isn’t even an option rn. Just sitting on him hurts. Sooooooo how do i loosen up my legs and give him the ride of his life?