Ex-girlfriend

BACKSTORY
Alright, so a week or so ago my exgirlfriend ends up at a party that some of my good friends at school were having (she lives in the town where I go to college). The reason that she came was because one of the kids girlfriends works with her. So we didn't talk that day but I went into her work later that week and we talked and then she came to a party that my house had last Friday. Well we talked pretty much the whole time and I kissed her on the cheek when she left. We talked a good amount over the next two days and then in a emotional lapse (My grandfather is passing and I've just been stressed and tired) I told her that I'm in this for the big picture.

She had recently broken up with the kid that she started dating a month and a half after I broke up with her. Well, I was in a bad place at the time and ended up trying to get her back but in trying to do that I was needy and tried so desperately to win her back. It got to the point where I pissed her off so much that we didn't talk for the next 10 which ended last week.

Up to this point I have been relatively good giving her her space and showing her that I can be there for her if she needs me but I'm not going to try to force it on her. I think I also made the mistake of asking her to hang out before I had to work both Saturday and Sunday morning which she didn't go for.

QUESTION
I just want to know what goes on in girls heads when they are broken up with that makes them so attached to that person. To the point where that person could treat them like complete shit but they still stick up for them and want to be around them. I've asked a few friends and they, without any real ability to describe why, said that that person is 'there other half and they can't see themselves without them' and 'that a part of themselves would be gone without them and someone else who made them feel exactly the same couldn't fill that'. I think that I've come to understand that girls don't really understand themselves what it is that draws them to that person. What I'm curious about are the specifics.

What is it that you feel like you can't live without. What are the feelings you have them they don't call you that makes you so upset (you should be pissed not fall for them more) why is that? Just all the feelings that you have that attracts you to that person.

Also, if anyone can describe the way that they moved from this situation to another person. What was it about that person that drew you to them? How were you able to let that person go?

I think its the whole 'want what you can't have' thing. It's not really that they can't live without the person, or they feel incomplete without them, maybe more obsessing over something that didn't work but really wanting it too. Knowing that it never could. Feeling rejected and wanting to prove yourself worthy. I don't really know but its something I'm going through at the minute with my ex. So it you come up with an answer, let me know!! There'll always be 'that one person' for everyone, it's not just girls. There's always that one person who you can't help yourself when around them, even if you really really wish you didn't care about them. It's an odd situation.

I think the idea of having a void and no-one else being able to fill it is more because everyone is individual, so there will be noticable differences between old boyfriends/girlfriends and new. Things that you'll like and dislike. No-one is perfect.

ex-girlfriends are part of the past.