I have been where you are. My DD is 9 months, and she is a joy to me now, but when I was pregnant I suffered terrible depression and couldn't imagine ever being able to say that.
Firstly, this is not your fault. It is not at all a reflection on whether you deserve to be a mum. You're not doing anything wrong. You're not a burden to your family. Antenatal depression is far more common that you probably realise. A lot of women suffer from it, and none of them are failures or horrible people - they're just going through an extremely difficult time.
What I would suggest is that you go back to your GP. It is safe to take some types of anti-depressants while pregnant. That's what I had to do, and I hated the idea at first, but I got to the point where it was that or I was going to harm myself very badly. They helped massively - evened out my mood, helped me to feel like I was coping, and got me through the pregnancy. If you feel like the doctor who signed you off work will not properly listen to you, ask to see a different doctor. Tell them how bad you are. Read out your post if you can't find a way to say it. Anti-depressants do take a few weeks to start working, but if you can get through that, they can really help.
Also, does your husband know how you're feeling? Have you been open with him about how much you're struggling?
I wish I could do more to help. I know how bad it is, really I do. I also know it is possible to get through this and feel happy again. You may not feel like that's true, but I promise you it is 