Really, masturbation stops you. It's a great tool when you're alone somewhere for a long and sometimes just to stay conscious. You know that feeling when you've not shot for at least two weeks? Everything looks so sexy, every girl looks to have something which arouses me, it's porn everywhere. I would reccomend doing it once a week only.
I use masturbation a few hours before the sex to make the sex longer, but sometimes it really fucks the things off an I am unable to finish. Try to catch the idea what's the biggest arousal for you. The mind that I will finish inside the girl is so hot to me that I must control myself not to think about that because after the pictures comes into the head, it's almost near the finish
Stopping masturbation also can be hard for the first days. It seems as a habit, but say yourself: "I don't need that now" and yoy should do very well without it.
Good luck