I'm so sorry about the whole situation. I can say with certainty that you would be having indecision and second thoughts even if you decided to keep it. Coming from someone who was in similar situation (as far as becoming pregnant and not sure about keeping it) I decided to keep the baby and I still wonder if I made the right decision sometimes. Is she getting enough food, love, attention, etc? Am I doing a good job? Is this a good life?
Every situation, person and decision are unique and I am sure you made the best decision for you and your situation. Don't let anyone else tell you differently. It will get better if you're open to letting it get better. Yours is not an easy decision to make but it does not mean you should let it haunt you forever. PP's have given you some great links and I hope you find someone to talk to to help you work out your feelings on this matter.
Good luck to you.