Masturbating the wrong way?

I'm a 15 year old male and I'm a virgin. When I masturbate, I use the common method of jerking off. However, there are a few things that concern me. When I jerk, my legs are really tight and close together and stretched all the way out instead of relaxed and spread out like most guys are when they are masturbating or having sex. When I try to do this, I can hardly get an erection and cannot cum. How do I fix this? Another thing is that I jerk off really fast. I don't mean I climax and cum really soon, I just rub my penis very rapidly an I'm worried that if Im having sex and it's going slower, I won't be able to keep an erection or cum. How can I change both these things? I'm really just worried about how the way I masturbate will effect my sex life.