How to last longer in bed?

I'm 19 years old and i feel like i should be at the peek of my sex life. Ive been doing the stop start technique and the Kegels but i can only last like 3-5 minutes. and then i have a down time of like 15 before i can go back into it. IS there anyway i can make the first time go longer? i mean i try to do a 100 sets of clenching and release on my Kegels. I can stop my orgasm but i have to stop everything while i clench and it frustrates my girlfriend starting and stopping all the time. If someone can give me a routine to follow with the stop start or something id really appreciate it. I heard somewhere your suppose to use your imagination and then gradually work your way to porn when doing the stop start technique any advice? thank you for your time.