Insert standard male disclaimer here
I don't want to sound like I am preaching a philosophy, though I suspect that it can be interpreted as such at some point by some people. I do not claim that this is right for anyone but me.
First and foremost, your decisions are your own. That you recognize the value of that choice, and don't have that choice forced upon you is a good thing. Likewise, you respect life and all of its possibilities.
There is nothing wrong with being selfish. Selfishness is recognizing your place in your world. In all things, think of yourself first.
You are not guilty. If you are guilty of anything its of having a conscience and looking at the consequences of your actions. That is certainly not a bad thing. Think of how much worse it would be if we never felt guilt, that everything we did was care free and without consequence.
Yes, it is possible that you made a mistake in the past. Chances are he did too. We all make mistakes. Everything we do touches other people. Having a child touches may other people's lives. You can already see it touching yours.
The past has been written and cannot be changed. Make the best of the present and the future.
It does no good for anyone to say "I wish I was on birth control" or "why didn't I make him wear a condom". What is necessary now is to look at what can be done here and now.
Do not, by any means look down upon yourself. You are not a "cheap half-rate slut". You are a person who is just as likely to make a mistake as anyone else. Things happen, sometimes outside of our control or plans. To dwell upon these things just mires people in the past, afraid to take a step forward or to make a mistake again. Take pride in your mistakes, they are what gives us assurance that there is indeed free will.
Whatever your choice is, it is the right one. But don't forget to take responsibility for it.
Too much knowledge never makes for simple decisions.