Mother Teresa always used to say that purity is the fruit of prayer. So, step number one is to deepen your prayer life. Step number two is to begin fasting. A fast means to abstain from something you like. It can be anything from skipping the ketchup to skipping snacks between meals, or a strict bread and water fast for a meal or a day.
In the eyes of the world, this advice probably sounds dumb. You want to avoid sexual stuff with your girlfriend and I tell you to cut down on the ketchup. But fasting does several things. It begins to train you in self-mastery, where you gain control over your body's desires. It teaches your body to be subject to your will, basically telling it that it won't die if it doesn't get everything it wants. Have you ever seen a spoiled kid at a toy store throwing a tantrum when he doesn't get what he wants? Sometimes when we spoil our bodies with pleasure, they become like brats who can't take "no" for an answer. Fasting makes you, as Saint Paul said, "your body's sternest master" (1 Cor. 9:27).
When you make a deliberate fast, it's also as if your body is praying. It lifts your heart and mind to God, and it actually makes it easier to pray. You can make this sacrifice as a prayer for purity, as a prayer of penance (to make reparation for the sins you've committed), or you could offer it up for your girlfriend. Whatever you choose, check out e5men.org for a great male perspective on fasting for women.
It's good that you've talked about your boundaries. But as you've seen, the talk is not effective unless it is put into action. So, one thing you need to do is to avoid occasions of sin. In other words, stay out of situations where you know mistakes will happen. Also, plan your dates in advance so that you don't just end up sitting around with her with nothing to do.
The bottom line is that you can talk about your boundaries, you can even write them down together, and even be in total agreement. But, these rules do not change one's heart. The desires will remain, especially if you want to get a little sexually aroused, and then slam on the brakes. Although it sounds strange, the more pure you are with her, the easier it will be to remain pure. The more you tease yourself, the greater the desires will be.
I know it's not always easy, but by keeping solid boundaries, you'll always continue to see the other as a gift. I remember on June 7th 2003, as I stood at the front of the altar, I saw Crystalina's shadow through the back door of the Church. The doors swung open, the violin and organ music played, and tears fell down her cheeks and mine. As she began to walk toward me, behind her veil, I had this overwhelming thought of God the Father's hands behind her, giving her to me as a gift. I realized that through our struggle for purity, we were blessing God by leaving the gift of each other in his hands until his time.
While we were dating, I could feel this calling somewhere in my heart for a deeper kind of purity. It was not that God was never satisfied with us, but that He was always inviting us gently to live a deeper commitment to him in the way we expressed affection to each other. He was always calling us to be more and more generous with him, in response to his generosity to us. It seemed like the more we gave God, the more we were able to receive from him. It seemed like he wanted us to be more generous with him for our sakes more than for his sake. Does that make sense? It was like he loved it when we love each other in the best way. He truly takes delight in the love his creatures share. This is because when we truly love, we reflect him to each other. In John 3:30, John the Baptist says, "He must increase, and I must decrease." If we really want to express love to someone, then this is the recipe. My job is to get out of the way, and let Jesus love Crystalina through me. By doing so, my expressions of love to her also become expressions of my love for Christ.
We're all tempted to think otherwise. We often imagine that we can achieve greater joy, love, and union on our own terms, apart from God. This is the original lie of Satan, and he hasn't had to come up with a new tactic since the dawn of creation . . . because we keep falling for it. So, draw near to God and he will give you your heart's desire.