Love you too!
I can't say, "if I were you". But what I can say is now that I'm 52, I have a lot of experience that a young person may not have. Whereas I agree with my man Musky, I don't think I could go so far to assume what you gf's motives are. People are complex and our issues can run deep. Women have instincts. That I do know. They sense things more than we do, sometimes. She may be sensing issues going forward because of chemistry, behavioral or otherwise.
Going forward, things get far more complicated after marriage, especially when you add more little lives into the mix. If you're having difficulties now, you can be sure you will have them later. Badly. You maybe being spared of something awful in your future. I would suggest a cooling down period. Let her go. Keep it cordial, not hateful.
It's likely that you'll find someone else and end up wondering why you held a torch for her so long. I've seen it over and over and over again. I don't mean to tell you what you're going through and what your future will be, so please forgive me for being so direct and assuming so much. Every situation is different. I just wanted to give you some perspective from the years I've lived.
Good luck to you.
Steven