Koshka, I couldn't read this thread and not say something. I felt something pretty similar to you when I found out at 11 weeks that I was expecting number 2. Panic. You;ve only just got your head around having one child, when a second one pops up and changes all the dynamics again. But mostly we all just muddle through (I know I did).
You are under huge amounts of stress and sound very tired. Your partner does not (forgive me for saying this) sound either kind or supportive, and is making you carry everything alone.
It could be that is not mature enough to deal with this, it could be that he does not have the right skills to deal with it, it could be a host of things. The fact is that he does not sound up to the challenge just now. I don't think you can really take him into the equation with regard to a possible termination; men rarely understand the long term implications of a termination and just see it as a quick fix- he needs to think it through.
MN is always wonderful in situations like these, but you sound as though you need concrete support and few good hugs. Do you have any RL friends you can talk to about this? Maybe even someone you don't realise is a friend yet, possibly a slightly older, sympathetic lady who could be a good listening ear? Have you spoken to your health visitor?
FWIW, we are all "shit mums" sometimes, particularly in times of stress. If you feel like one, then it's extremely unlikely you are any more shit than the rest of us. Children are remarkably resilient, and your little will be just fine if he has a mum who cares about him (as you obviously do).
Please please please keep trying to find a sympathetic ear. If your GP is so crap, find a different one. He sounds like an arse. What you need mostly is the chance to really talk through all these issues with someone calm and sensible and measured, so that you can come to the decision you need. Keep posting on here as well- you get a range of experiences to draw on.