@DMKP,
Bullshit.
You are not married to this gal.
I get the feeling you are young - very young. Do you want to be dealing with this crap for the next 50 years? Seriously?
Life does not have to be this way, and you are not shackled to her.
Do you know how many relationships I was in before I got married (at age 29)? He was my 12th, for guys I dated a month or more.
My husband and I have been married for over 23 years, and together for over 26. We love and care for each other. If I had stayed with most of those other guys, I would have been in divorce court at some point.
Yes, you both promised. I get that. But you can both breach that promise. It's okay. It doesn't mean you're bad people or you are incapable of committing or you're immature or anything of the sort.
It means that you care enough about each other to let the other person find someone better suited to them. It means that you care about each other's happiness.
Dating is an experimental time. It is rarely forever. It isn't meant to be. It's how you figure out what you like, what you despise, and what you'll put up with. It's how you learn to compromise, and how to take a stand. It is not where you learn to just let **** happen to you forever because of some vague, unenforceable promise.
You can both end this, and I bet you'll both be relieved.