My question is, how the fuck do you cum on dope? If you're doing dope on the reg anyway, find that amount where you can still get hard, but won't cum easily, and go to town. If you're not doing dope daily, put that shit down.
When you jerk off, don't make yourself cum immediately. You'll condition yourself to cum fast.
Thirdly, if you cum fast, don't make it a big deal, say something like, wow, you feel absolutely amazing and I couldn't control myself. You're just really fucking sexy. Act like it's no thing whatsoever, go down on her, or just recharge, get your dick hard again, and you'll fuck longer round 2. Or...just use a condom? One of the thicker ones?
I'll tell you right now though, dope fucks with your ability to pee and to cum, and when you're not on, you'll cum ridiculously easy. withdrawal, even mild, makes your dick stupidly sensitive.
Rest assured, you probably won't have this problem when you're 30. Another thing, I actually find that going from non-sexual to penetration pretty fast makes it easier to last longer. in other words, if you've had a boner for 20 minutes, have been thinking about fucking, or have been making out/engaging in a shit ton of foreplay and you've already got some precum dribblin', and you're predisposed to premature ejaculation, you'll probably cum really fast due to the anticipation. So, try and move from point a to b a little quicker, once in a while anyway, and see what happens.
Also, you should know what movements, speeds, etc. make you cum. For me, counterintuitively, if I speed up and beat it down, I'm less likely to cum than if I slow stroke and make it more...intimate. That's me though.