Why don’t I last long?

When me (M19) and my girlfriend (F21) have sex, I cum within like 30 seconds of it. I’ve heard of this being something that’s frowned upon (for good reason) so I always make sure she finishes before I fuck her. It’s always like I cum, and then my tip is super super sensitive, to the point where I can’t put it back in her. But this one time I came and was fully prepared to not be able to keep going, but I was actually pretty hard still even after that and we kept going for about an hour and both agreed it’s the best sex we’ve ever had. I want that every time, how do I achieve that?