I'm confident that if you chose abortion, it was for a good reason. You shouldn't have to feel bad about it. A woman should feel bad for having children when she can't properly take care of them or give them the love they deserve. Not for preventing a situation like that.
As for the "what if's"...there are always going to be "what if's" in life. Think about simple decisions you make every day...If you drive, I'm sure there's been times when large trucks passed by you. What if one of those trucks accidentally swerved into your lane and hit you? You can turn virtually any situation in life into a "what if".
What if your pregnancy would have ended suddenly in a miscarriage because it wasn't developing properly? That's always possible in any pregnancy. You'll never know if that would have happened, but doesn't that make it a "what if"?
And I don't know your situation, but if it's money related...what if you kept the pregnancy and had a baby later this year, but you couldn't afford to buy things it needs? Or if you just weren't ready to be a mother...what if you'd had a baby and weren't sure how to properly take care of it, and you ended up harming it in some way?
You can't let "what if's" rule your life. There are so many of them all the time that they would drive you crazy if you let them! You can't take back what you did, but you can learn from it. You just have to think positive, and remember that you did what you did because it was your best option at the time.