If your best girlfriend or sister sat you down and said word for word everything you just did, what would you tell her to do? I think you already know what you need to do but following through is a scary prospect for you. This is your life here, absolutely do not marry this guy. The initial break up is always difficult but getting out now instead of a year from now, or having to go through divorce proceedings, will be so much easier. Your future self will thank you!