After the first few times with a girl it becomes pretty easy. Foreplay plays a big part too, have her go down on you first and return the favor etc.. Pounding for over an hour? I doubt any girl would be into that lol. When that happens and you don't think you're gonna be able to finish, slow it down and do something else, and build it back up to that point. This will make her want it more as well which always helps.
You're young dude. Your worst enemy in this is that you are so worried. Take it easy and don't be hard on yourself about it.