Why did she leave me for him again?

I think a lot of girls who have low self esteem will feel like they don't deserve the nice guy. He may be wonderful, but she will feel like she has more in common with the bad guy and gravitate towards him.

Don't go for a girl in "re-bound mode", old habits die hard.

Yeah I do the same thing. The good thing is you are starting to realize there is a problem. Im starting to think that I should go into a relationship believing every girl is on the rebound. I mean it sucks but I'm tired of being hurt by girls who lie and really don't give a sh!t.

You need to identify what you are really looking for in a relationship.
and pay attention to the following things:

Does she call you or do you always have to call her?
Does she like to hang out with you?
Will she make plans ahead of time and stay committed?
Is she always on time or does she tend to run late often?
Does she express her feelings?
Is she willing to hangout with your family?
Does she hug you without you having to ask?

Take notice to these things. It is important to write your answers clearly down on paper.
Read over your answers and make a decision. If most of the answers are 'No' move on. The girl has commitment issues. She might not be going back to her ex, but she doesn't want to commit. She will probably say she is ready. She might even say that YOU aren't ready. (that happened to me) In that case she is projecting herself onto you. I realize now that women do a lot of projecting because they don't know how to express their feelings. I know this because I am honest and yet still confused. If you know you are being honest and yet you are still confused then there is a problem. Hope this helps

Stop being the "nice guy"

It sounds like you are running into the ones who like bad boys, but choose nice guys once they are treated poorly.

Now there's also the nice and walked on guy, and the nice-but-stern guy, like: Jon Crier/ Allen from two and a half men (who gets walked on), and then Jason Statham/ the transporter- he a nice guy, but don't break his rules, just don't. But even he broke one of his own rules, so is human. But he didn't let no one push him around without pushing back. Jon Crier/ Allen - just gets pushed around.

So I say, you can be nice, but communicate/act with a purpose.

And this is another example- Malcolm X said when he was a teenager: " I can talk amongst the pimps, the players, the hustlers, the gang bangers, the pushers, and get the respect I deserve. Now when I write a letter to a fucking senator, I get not one fucking response. No Respect..." ... well, why?

This is before Malcolm X actually learned how to use his words wisely to get his point across. It was a different type of communication taking place. He wasn't being heard, so he had to adapt what bothered him, into real rational debates to get his point across. Like being nice, but also being able to have some kind of alternative way to show or tell that you are not to be walked on.

As hard as this is going to sound what is the one thing that is in common with all of these girlfriends? You. There may be some other connections about them. I would like to give you some good advice but some more information is needed. I bet if you think about it, you can probably come up with some great advice on your own about this. Like were are you meeting these girls? Did they come from an abusive house? These sort of things, if you really think about it I am sure that there is some sort of connection