Glory to God that you have repented and are seeking Him! If you have repented of your sins, there is no need to think of them any more. I'm almost of the mind that beating yourself up over the sins of the past is in and of itself a sin. I say this not to worry you, but to make the point that constantly berating yourself on mistakes of the past causes you to fall into despair (trust me, I've been there, the accompanying feelings of worthlessness and self-loathing are neither healthy nor pleasant). Rather than looking back on your sins with regret, look to God with joyful hope and expectation, seeking to deepen your relationship with Him!
I've also been a chronic masturbator since I was 14 (I turn 22 in a few days). What I've recently found to be a more helpful approach with just beginning to cut down on it/eliminate it is to say "You know what, I have too much respect for myself and for God to stay weak and fall back into this. I want to be strong and persevere, so I can keep growing spiritually." And when the temptation comes, I tell myself, "Be strong and pull through!" It hasn't been 100% improvement yet--I'm still tempted and I still entertain the thought for a minute or two (which you should NOT do, mind you--that's the textbook way to get ensnared in the same old sin again, take it from me!)
Better people will tell you to pray when you're tempted, and they're right, but for where I'm at in my spiritual journey, the above approach works best for me personally. You can figure out what works best for you using your own powers of discernment. When you're tempted, you might try making the sign of the cross, or if that isn't your thing, say a quick, five-second prayer, and repeat it if need be ("Holy God, Holy Mighty, Holy Immortal, have mercy on us", "Lord Jesus Christ, Son of the Living God, have mercy on me, a sinner" or simply "God protect me" or "Bless me, O Lord" are all great examples).
One last recommendation I would give you is to find a community of believers that you click with, whether it's someone's house church, a Baptist church, Methodist, Lutheran, Catholic, Orthodox, Anglican, non-denominational, whatever, just find some church that you can start going to, and begin attending worship, Bible studies, get to know people. Having a support network will be very helpful as you begin to deepen in your faith and in your relationship with God.