(Need BBW Help) I CANT RIDE FOR SHIT

Every time I try to ride my boyfriend, he always end up flipping me over and taking control. I just can't do it! My legs get tired to quickly and I end up freezing up. I tried getting on my knees and moving back and forward but I feel like I'm hurting him and it gets boring. I tried squatting over but My legs cramp up. Do y'all have any tips? I literally cried about this. Idk what to do.