leigh1991 I'm going to disagree with a lot of the posters here.
I'll admit my bias here - I had an abortion when I was much younger and I have always regretted it.
Now you can have a look on the forum and see comments from people with all kinds of experiences - some with regret, some struggling to deal with their decision and some not regretting it at all. But abortion is a HUGE deal. It shouldn't be based on any relationship you're in right now, because only one person has to deal with this decision for the rest of their life - is you!
This isn't about your boyfriend or the guy you slept with - this is a decision as to whether to bring your future son/daughter's into this world. Now I could lay it on thick by saying you should look at the conception forums for people struggling to conceive or give anything for having a baby. But all I want you to understand that this is a huge, life changing decision whatever you decide.
You seem to be so wrapped up in your relationship problems that the focus should be on the baby - whether you are ready to be a mum, whether you are ready to be a single mother if needs be, whether you are willing to forgive yourself for the mistake you made and be strong enough to deal with the consequences.
Your current boyfriend - this relationship may or may not work out. If your name has anything to do with your DOB, then you are still very young and have plenty of time to find the right person. I'm sure there are lots of private and personal reasons why you cheated on your boyfriend - but ultimately I don't think committed and happy people cheat on each other. Surely there must be a reason why this happened?
I agree with some of the other posters that you do need to tell your boyfriend what has happened. If he doesn't hear from you, he could find out from someone else. He could cheat on your or decide to end this relationship anyway - in a few months, years down the line. Again there are loads of threads on the forums about people who have had affairs and whether to admit it or not admit it. I am of the opinion that a good relationship is based on trust, respect and faithfulness. Do you really want to be in a relationship where you don't have trust and respect?
On top of that, you are putting his health at risk by continuing to have a sexual relationship with him after having unprotected sex with someone else. Both of you need an STD check, not just for his sake but also for health of your child as well.
Whatever you decide OP, I wish you the best of luck. If you don't feel you can continue the pregnancy, make sure you get as much support from your family, counselling and that you have all the information and resources needed to deal with the emotions that come with it. If you do decide to go ahead with the pregnancy, work out a plan of action of how you will bring up your son/daughter with AND without your current relationship. That way you know you will be able to deal with whatever comes your way!