The real question here is not why men (and women) do this, but how to move on. I just wanted to tell you what has and hasn't worked for me, having been in a similar situation and now a single parent.
I hate self-help books, but read 'Why Men Love Bitches' by Sherry Argov. Guess I was looking for answers online and this book seemed to crop up a lot. It's not about being a bitch...but it completely changed my attitude. What a lot of mistakes we women make...it just told me how to be strong and in control without lots of annoying psycho-babble and jargon.
Don't expect to be alright overnight. It takes time. After two years I still get dark cloud days. The days when you just can't imagine ever being properly happy again and can't be bothered even to brush your teeth. I'm slowly figuring out what works for me and I think for most people:
read baggagereclaim.com....this was the start of my recovery from the pit of hell (Samaritans, Prozac, suicidal thoughts, etc). My the author is one amazing woman. Expect to think "does she know me?"
exercise...it just works...those endorphins make your skin tingle and give you a rosy glow
talk to a handful of close friends, rather than everyone. If you're always talking about your crap, you never get away from it.
Drop a load of cash at amazon buying brilliant DVDs...dramas, comedies. I was never a TV person, but this was the only way I could forget my crap for at least an hour.
Don't blame yourself...we women are so great at this. Read the bitch book which will make you realise that we must be strong, even if we don't have a job/husband/partner/money, etc. My attitude changed almost overnight reading this.
Accept that maybe you won't meet someone to be by your side. This is hard. But easier when I talk to married friends (most of them) whose husbands ignore them/have no respect for them/hit them/do what suits them. I would love to have someone, but right now I am just appreciative of my loving kids and incredible friends with sore ears and wet shoulders. The grass is not always greener.
Figure out what YOU want to do. I kept trying to do the things other people wanted me to do/what I thought I should do. That was so bloody stressful because I didn't want to do those things (get a certain job/move house/dating websites, etc) and I would beat myself up about it. It's my life...that was a revelation!
Invest in your friends and worthy causes. I helped friends move house/collect kids, etc and started helping out at school. Before I was too lazy...now I know what a wonderful feeling it is to help someone out, even in the smallest way.
Hope these help. I was a horrible mess at my lowest. I still can't see the future, but I keep trying to look for one. You will get there too.