Sad, i am so sorry you are having to go through this. I had an unplanned pregnancy when my DD1 was 15, i thought we had done with the whole baby thing and was just getting our lives back, then wham, stupid me, im pregnant (not a contraceptive failure, a common sense on ). I did consider a termination breifly, but decided to have the baby - of course i love my little girl dearly, she is the light of my life, but yes, it is soo hard, on top of all the financial problems caused by me being unable to work, i got PND. I still don't regret my decision, well for me, it wasnt an option. This was because my eldest was 15. To be brutally honest, i think if i was in your situation i would be the opposite, i would feel, as you do, that i have no choice and that i would have to go for a termination. Something i have always said i would never do. But this doesnt just affect you and DH, it affects your other children too. FWIW, i think you are making the right decision i really do.
Something bothers me though, and that is your entirely natural feelings about it, your comment about wanting to never forget it and wanting to punish yourself. Sweetheart, please dont feel this way, you have to make the decision for your family. I think you should get some counselling, not to talk you out of it, but to make you ok with it in your head BEFORE you go through with it. Does that make any sense?
You are a mother, and sadly, as a mother we have to make some decisions that are painful, for the sake of our families.