Hi,
This is my own opinion so please do not be offended by what I say, but sounds like she is playing "mind games" with you. I think you both must make the decision right now not to be together while she is seeing this other guy, it is not fair to you to be the "part time" lover or the "fill in". Do you think this is maybe becomming an ego trip for her? After being steady with someone it is almost impossible to resort to just "seeing" one another, plus I personally couldn't emotional stand it, it would become painful. It is different if you both were only dating from the begining, but it sounds like from what you've expressed that it was more. It may be hard for you at first but try not to be so available to her right now. Just my opinion, and yeah you probably aren't looking for any right now while you are feeling down, sorry I just hate to see anyone "hurting" while the other person seems so insensitive, I've been there, it is good you weren't married and had her do this to. Don't be taking all the blame either (that's what my past therapist told me with my situation) and he was right.
I hope things start looking up for you soon, you are a nice guy and you know there are some nice gals out there!
Take care Dr. John,
"darkeyes"