How i can stop masturbating? is there any way?

You;re doing what comes naturally. Stop worrying about it. If you're concerned about attacking your gf, and you know when the two of you will meet, masturbate just before you see her. That should satisfy your desire for sex for a while. If you're in the 15+ yr range, your hormones are raging and it's difficult to stop. Enjoy it while you can. Apparently your gf really turns you on. You're really lucky.