Well, You didnt bone her. Done worse. Everyone is a horrible person at some point in their life. Its part of growing up, something that you never stop doing.
Fuck it, I used to be a user who fucked around with whoever I could fuck around with. Cheated on people, fucked people, manipulated my way into a relationship out of boredom, yeah. Am I a horrible person?
Nope. I was a horrible person, I have been a horrible person and I have done horrible things. But I am not a horrible person.
Quite the reverse, I am a good man. Incredibly so. Someone recently joked that I was attempting to apply for sainthood, what with the whole "Atoning for being a massive penis during my late teens" angle. I have a beautiful girlfriend who I will never cheat on, I do not use any more, I don't fight, don't wank, don't watch porn, don't smoke... You get the idea. I have deliberately (And with little effort) made the choice of being a good man, devoid of all of the character flaws that made me incredibly interesting and leaving the decent human that was hiding behind it all. And that guy is universally loved these days. Hell, outside of self serving rants like this I have even managed to tone down my arrogance.
So whilst you may have opened up with a way of bearing your soul because you have been a horrible person it looks like I accidentally went off on a rant about how fucking awesome I am now. The main points of it all were simple enough though. Doing horrible things does not make you a horrible person and no matter what you have done one can atone for their mistakes and become a decent human being, it takes very little effort.
Well what you did sucked, but at least you handled it the way you did, rather than cheat on her and that was really good of you. But you ended a relationship in which you were not happy, and that is a good thing. You weren't malicious or anything but doing it over an IM is pretty damn dickish of you, but just do not do it again, learn from this experience.
I don't see anything wrong with this at all... being involved with people on any point on the emotional spectrum is not only experience for how to live life, but also is invaluable for self realisation and self analysis...
What you did was in your subconscious at the time... only with analysis and hindsight you realised that you felt that what you did was wrong, and now you know what it was, how it feels, and you know what to look out for and situations to avoid in the future! Relationships are primarily selfish things... You try and meet someone to engage in a relationship to fill your own needs first, and then eventually, if the relationship goes far enough, you focus on yourself and the other party. Some people would take what you just learnt and do it again out of spite or a desperate attempt at attention seeking (from an easy target). The very fact that you feel bad and don't want to do it again means that you are in-fact a nice guy, and you should feel good for that!
On her side she will have also learnt (hopefully) from her experience... she should now realise to look beyond her own feelings and see what another party feels too, and not to be blinded by her own needs! She will also have learnt what it is like to be in a relationship with a guy who is not really that interested, and a bit different... this means when she finds someone who reciprocates her own feelings next time, she will not only realise this, but she will also enjoy it more, as it will feel that so much better!
We have all been there fella... doing something we later regret is part and parcel of life, but should be viewed as a complete lesson identified, and lesson learnt, and not something that will just haunt you for the future.
And thank the fact that you didn't complicate the situation with sex...
So remember: Learning point... move on... remember not to repeat next time!
You should also save your OP as a notepad file on your computer for future reference! One day you will look back on it and wonder why you got so worked up about something that will then feel so insignificant! :P I save details and 'late night truth essays' about my relationships when I hit a rough patch for future lols! It's great fun!