YOu are WAY too young to be thinking of getting married, and the stress over this forced intensity is probably helping to distance you from this girl.
You'd better man up and tell her you aren't thinking of getting married till you are at least 30, and you don't want to be pressured about it. Then you can see how SHE reacts.
Honestly, I think most people at your age don't end up staying with their boyfriend/girlfriend. You are just too young to make a choice that will affect the rest of your life.
Relax... I'll need some information first. Just the basic facts - can you show me where it hurts?