Och poor you. No useful advice but seriously try not to torture yourself. So many emotions and hormones going on, the abortion, your 6 month old and the loss of your little boy which is still relatively recent. Plus you must just be so tired from having a small baby and the stress of the decision for the termination. You certainly haven't let your children down in fact this difficult decision I'm sure was made with them in mind. Plus guilt is such a insidious emotion sometimes - it makes you think you've made the wrong choice when you know you haven't and it creeps up on you when you are at your most vulnerable. This is a trivial example but I left my marriage a few years ago and I know it was right but it didn't stop the crushing guilt then and sometimes now. I felt so much responsibility for him, my children and parents who were all very upset to the point of thinking I should've stayed to keep everyone else happy. Guilt messes with rationality I think and makes you think the darkest thoughts about yourself. Usually when you are a bit tired and rundown I find. Just remember - just because you feel guilty doesn't mean you made the wrong decision.
Good luck Kate, take care, I know you are going to therapy but look after yourself physically as well. X