Totally understand what you mean. If a person were all bad it would be much easier (or at least in some cases). If you never found anything she said or did funny, if she didn't show some compassion and understanding sometimes, how much easier it would be to walk away.
The fact that she was embarrassed by resuming your relationship after breaking up for a while is another huge red flag. Looking outside yourself for confirmation that you are "ok" is never a good thing. Of course we all do this from time to time, that's human nature, but this is different.
You know. As you said everyone is "expecting" you to get married I'm thinking she does too?
The longer you drag this out, the crueler it is. I think you have already made up your mind, apologies if I'm wrong about that.
If you have, the kindest thing you can do is end it, with compassion, as soon as possible.