Messed things up a bit...

Right, recently, I blocked all contact with a girl I really really liked, and now I can't stop thinking about her, and I want things back as they were. How can I go about this the right way?

Some background:

The girl and I had an LDR for quite a while.
She phoned me up for my 18th, and told me she loved me, and wanted to meet up soon.
I found out about 2 weeks ago that all the while she'd been with other guys, even though she'd been telling me to keep away from other girls.
I told her that it would be better if we went our own ways, cause she obviously didn't care; I'd had no other relationships except her, while she'd been going around with a load of other guys.

So basically now, I'm starting to regret it.
Even if we weren't dating, or whatever else, she was a really great friend, like the best female friend I've ever had.
I want to get things back now, but I've deleted her number from my phone, deleted her from MSN, bebo and myspace.

There's two reasons why I possibly want her back:
1. I genuinely love her. I mean, I've 'gone off' her in the past, but I've always gone back; I've never gone so far as wanting her out of my life before though.
2. I promised her "on my life" that I'd visit her, and every time I think about her I hear her saying "Do you promise on your life?" Silly, but still a viable reason.

Please help, anyone?

Has anyone been through something like this before?
How could I get back to the really good terms we were on?
Or
Should I just forget her, and keep trying to move on?