And laziness is taken to a new level...sex sucks because you have to do 'work'
Try not jerking off for a few weeks, then have sex with a condom on. It will feel better. I was never a fan of how sex with a condom personally, so you have to think of ways to increase how sensitive your dick is, and cutting out masturbation is #1 on the list. Sex without a condom does feel infinitely better, but best wait on that practice until an accidental pregnancy is something that falls into the realm of acceptability.
And don't be afraid of having her get on top, she isn't going to crush your manhood, that is kind of a silly thing to worry about.