Right I'll just get down to the point, I've been with my girlfriend for 3 years now. But only recently during sex I penertrate her with a 100% erection, but after a while it goes down to around 50% or even less while we are still having sex, this is very confusing for me seeing as she is a very attractive girl, and I was wondering if someone could help me find out while I can't stay erect while having sex with her

Of course you don't want to hear it, but 15 year olds have no business having sex, That first in itself part of the problem.

You have stress issues of getting caught, her getting pregnant and of course the entire issues of sex at an early age.

So to fix this, take two large glasses of water and wait 3 or 4 years at least and most liekly the problem will go away.

You are not going to get any more help here than Fr_Chuck has given you. As I've said before, NO ONE should engage in sexual intercourse unless they are financial and emotional able to have a child. And you aren't ready. In addition to the risks of pregnancy, you are also risking jail from a statutory rape charge.

I'm not going to suggest that you cease all sexual activity, that's closing the proverbial barn door. If you insist on being sexual active, there are ways for you and your girlfriend to pleasure each other without intercourse.

You are tooooooooooo young to worry about, much less be partaking in intercourse.

sex = parenthood.

Unless you are financially and emotionally ready to be a father, you should not be having sex.

PERIOD!!