While I haven't been in a similar situation. I definately know that feeling. It's time to leave. When you have that nagging feeling you, it's telling to not to do it. That feeling is often times right and it means you should go through with it.
DON'T EVER MARRY BECAUSE YOU BELIEVE YOU CAN'T DO BETTER.
Marry because you believe she is the best. And the way you are talking, She's not Mrs Right.
I was once like you, Inexperienced in relationships. I believe I couldn't do better so I stuck with the first girlfriend I had. She then became a complete and utter bitch to me, and nice to every other guy. And I just took it. She then broke up with me.
After a while, I began to understand it really was for the better. And I've met tons of girls that I actually seemed much more interesting that my last girlfriend.
So please get out of that relationship and experience different girls. Some may not like you, but others will.