I get limp after ?

okay so im 18 years old, and about 3/5 times that im messin around with a girl, i usually get too much into and exert all my energy and become tired ( for a long time etc) and i guess it tires me out and when we progress and start taking our clothes off about to have i become flaccid and have a hard time getting it up. i can a little but im so tired from i can only get semi's and its hard to . ive fixed this a few times with lube and got hard as a rock. any tips?