Well, sweetheart, it's nice that you want to give her a satisfying time. Is she on board with the "it's time we do the real thing ... I wanna have sex with her within another 6 months or 8 months" plan? Or has this been sort of unilaterally decided by you? My first suggestion would be to talk with your girlfriend about the plan, or you might find yourself having sex in 6 months or 8 months with thin air. The check needs both signatures, as my now-husband once said to me. (One of the reasons he is now my husband. A considerate guy.) The reason I mention this is that you didn't say "We have talked about it and we agree that we should be having sex in 6 to 8 months."
You're a teenager and obviously hormones are pretty much in the driver's seat. It isn't too surprising that you come fast. I assume when you are making out, you don't bother with a condom? You might be surprised how much longer wearing one will make you last. And of course in the I wanna have sex with her within another 6 months or 8 months plan, you and your girlfriend intend to use condoms along with her being on some other form of birth control when having sex, right? (Riiighhht?) My friend Wayne, who paid child support every month from age 17 to 38, would be glad to explain to you the importance of birth control. As would my friend Olivia, who got herpes from her first sexual experience.
Anyway, it is quite possible that merely wearing a condom will take the pressure off in both physical and psychological ways. If that does not do the trick, you could try masturbating prior to having sex (but you do have to be ultra careful about your condom and her birth-control if you have sperm in your urethra when you enter her vagina). Get some sex education about when and under what circumstances pre-ejaculatory fluid can carry sperm, and get some education about STDs, and think deeply about what it means to be a gentleman around sexual issues with your girlfriend. No bragging, no strutting, no pressuring her, and absolutely no unsafe sex. If you get tired of the relationship after having been sexual for a while, let her know and let her go, do not continue to sleep with her as a booty call. Basically, put her concerns as high as yours, and her first concern has to be preventing pregnancy. In short, treat her properly. This is your chance to learn to be a good boyfriend in a relationship that is quite grown up. Lessons practiced now will stand you in good stead for the rest of your life, and if you learn well, all your future partners will love you.
ps -- 15 minutes is not usually necessary, you can let go of the reins once she has had an orgasm or two.