No, this isn't what OCD is, so don't worry about that. Yes, it sounds like it would help you to talk to someone about it -- couldn't hurt. Guilt is built into humans to teach us about things we did, but when it lasts and lasts and lasts, that's not learning anymore. It's natural to feel guilty or sad about difficult things that happen in life, but when this thinking is so pervasive it interferes with your life, then it can't hurt to talk it out with a professional. This goes beyond grief or sadness and becomes something chronic, and that isn't helping anyone, especially not you.
Aww, sweetie, I'm sorry you have something that feels like a secret guilt that you have trouble shaking. Intrusive thought is a pain! I would think that working with a therapist would be an excellent choice for you.
Abortion is one of those things that people try to bring a lot of morality into and it is very political. Sweetie, you are entitled to make the legal choice you had that best suited your life at the time. Nothing changes the trajectory more of a young woman's life than having a child unplanned very young. You were entitled to the choice and it was what you felt was best at the time.
How is your life in general? Are things going pretty well?
Anyway, I would say you could benefit from talking to a professional about this. Especially if you feel you can't tell anyone else. We're here to talk to! hugs