My advice is that you should stop testing your limits. In other words, focus on staying far away from situations that give you time alone or in a place where passions can rise. Hang out with others when you are together (family, friends, the mall etc.) If you truly love each other and are ready for marriage set a date instead of burning with sexual desire outside of the bounds of marriage.
Most of all, utilize one of God's greatest weapons of warfare: His Holy Word. You don't have the power to stop all by yourself, but Christ does. Start memorizing scriptures which forbid pre-marital sex (which includes, by the way, all the other sensual stuff you find yourselves doing without actually going all the way). Examples include:
"Flee fornication. Every sin that a man does is without the body; but he that commits fornication sins against his own body.. know ye not that your body is the temple of God?... Therefore glorify God in your body, and your spirit, which are God's" 1 Corinthians 6:18-20 [memorize even the parts I left out to save typing time]
"Flee also youthful lusts...but follow righteousness out of a pure heart"
2 Timothy 2:22
"Marriage is honorable in all, and the bed undefiled: but whoremongers and adulterers God will judge" Hebrews 13:4
If you can find more, do so and start sticking them up in your individual bedroom walls and mirrors, cars, wallets etc. - any private spot you frequent a lot. Put them in your mind and hearts by reading them each time you pass by them. I speak from experience. I did this when I was in college and dating to find the right one and God's Word has amazing staying power!
The writer of Psalms 119:11 reminds us that: "Thy word have I hid in my heart that I might not sin against thee". The Word of God gives us power to overcome. It is not enough to just flee from sin, but we must also create a fort around us by drawing close to God. (see James 4:7-8). It admonishes us to draw close to God while at the same time resisting the devil.
Another option is to end the relationship for now if you know that you are not ready for marriage and instead focus on building your careers or whatever else you feel you need to accomplish beforehand, so as not to put yourself through unnecessary agony for the time being.
God Bless You.
