I totally understand what you are saying. It's difficult to maintain boundaries in a dating Relationship, just as encouragment let me tell you, It's WELL worth the wait! It is well worth doing what needs to be done to keep your relationship pure in all aspects. I don't have much advice on "how" to keep yourself pure, though I do have some advice that i was given while dating my, now, husband. This takes participation on both parts and dedication. Simply put, we were told, anything that causes an errection, stop. Easier said than done, right? If it is when you are alone, cuddling, creating those feelings of sexual intimacy, maybe those need to be cut down? We used to spend a lot of time WITH people. We both committed ourselves to control our 'fantasy' lives as well. It is also helpful to sit down with each other and make a solid commitment, 'how far is too far?'. Define your boundaries. If something causes you to stumble, stop doing it. I don't know if it gets much more 'practical' than that.
Don't give in JUST becuase you are GOING TO (Future tense....note...) get married. Everything you are are GOING TO do doesn't always happen. Even then I have spoken with people who feel immensley guilty for having "cheated" themselves. Make your commitment and stick to it. It's worth it, well worth it. Don't believe what the secular world labels as sex.
In closing I once read something from a Pastor somewhere, "When you start thinking about sexual things, having sex, etc... think about Jesus." Redirect, redirect, redirect.
I hope you are successful in maintaining the purity of your relationship, and if you can find someone perhaps and accountability friend would be good?