Hi I can sympathise, before I met my current boyfriend I really struggled with staying faithful to people. I didn't like becoming close to one person so they could hurt me so I would cheat to hurt them so I felt better. I use to also run away from the problem and not be straight forward and honest with the person when I had cheated. Now speaking from experience, my ex boyfriend cheated on me a lot, and because I honesly cared for him and hoped he wouldn't do it again I would forgive him, the worst part of the cheating though was when he didn't tell me himself and I had to find out from the girl he did it with, one of his friends, a text message or something. It made me feel like nothing because he couldn't even be bothered to tell me himself, and it hurt because not only did I feel betrayed but I felt like I was living a lie, cause while I carried on happy and natural he knew in his mind what had happened. However, my current boyfriend went on a night out when we were seeing each other - so we weren't actually official - got really drunk and ending up kissing a friend who decided that night would be a good night to admit she was in love with him, however, he came straight home after, the following day he met up with me and told me the whole thing. He told me he was going to leave me because I deserved better and he was ashamed of how he had cheated on me, 2 years down the line, we are official and have been together this whole time. He completely deleted that girl from his life to the point where she walks down the street and he looks through her like she's invisiable. What made me stay was that I realised if he was willing to be honest and admit what he had done, and that he wanted to finish us over his illtreatment of me, then the chances were he was unlikely to do it again. Now there is two sides of opinion from someone whose been cheated on, I don't know if this is much help or if your mind is made up that you can't tell your partner.
On another note, like I have already mentioned, I too have cheated therefore I am not an innocent party, although I have never cheated on my current boyfriend. When my ex boyfriend was out cheating on me I did it back to sort of prove a point. I told him once and he stook by me like I had him, however, another time I hid it from him and instead tried to pretend that it had never happened. The guilt ate at me inside and somehow or another he guessed something was amiss and when he asked me if I had been cheating on him I couldn't hide the mistake I had made.
In your situation owning up could loose your partner but so could hiding it, so its a tie between being honest and hoping they appreciate it, and keeping the secret and hoping it doesn't haunt you, and that if it does they can see past it.
As for the abortion and the terrified of being pregnant again, have you considered any contraception? There are a lot of good methods out there and as long as everything is done correctly pregnancy isn't likely unless you want a baby.
Hope some of this helps you :)