@clairindespair - I hope you got some sleep last night - sleep deprivation makes everything so much worse.
Now - I am old enough to be your mum, so I am going to be firm in a mum way.
STOP putting yourself down - you are doing the best you can in incredibly difficult circumstances. And you have done the single biggest thing you could do - you have asked for help - from us, from your mum and from the GP. You should be so proud of yourself for that!!
I am sorry that a tiny minority (splattt) on here has let you down, but I hope you are heartened by the fact that most of us support you, and by the practical help you have had from your mum and the doctor.
I suffer from depression, anxiety and low self esteem, so I know how easy it is to listen to the negative thoughts in your head, and how hard it is to believe the good about yourself, but I hope you can, in time, see yourself as we are seeing you - as someone who is struggling, but is doing their absolute best!!
You don't have to be a perfect mum - none of us is. We all do our best for our children, but we are flawed human beings, so it is never going to be perfect - and it is far too easy to beat ourselves up for not being perfect - and then you end up in a vicious circle of beating yourself up, feeling bad, coping less well than before, and beating yourself up again for not achieving perfection.
I read a book before I had children, and have read it again many times since. It is called "How Not To Be A Perfect Mother" by Libby Purves - where she shares her own experiences and those of her friends 'from the coal face of motherhood'. It is funny, and human, and has helpful suggestions from Libby and her friends. If you'd like to read it, I would be happy to dig out my copy and post it to you - as a loan - you can send it back when you don't need it any more.
And just to try to give you a bit of a laugh - here's one of my stories about my less-than-perfect mothering skills:
When ds1 was in the midst of the Terrible Twos, I had some dreadful times with him screaming for hours on end. One particular morning, it was not yet 9am, and he had already been tantrumming for nearly an hour - having had a 90 minute tantrum the night before. I was absolutely beside myself - and rang dh in tears, saying I just couldn't cope any more.
Whilst we were talking on the phone, ds1 stopped screaming, so I finished the call and went to investigate (screaming is horrible, but silence, from a toddler, is a Very Bad Sign) - and I found him standing by the kitchen bin. Dh and I had had some tortilla chips the night before, but hadn't finished them, and had binned the leftovers - and ds1 was at the bin, eating the tortilla chips from the top of the bin!
But that is not the Bad Mother Moment. This is - I looked at him, and I knew, for certain, that if I took him away from his delicious bin-snack, he would start tantrumming again, and there was just no way I could face that prospect - so I let him carry on eating!! As soon as he got bored and wandered off, I emptied the bin, and found somewhere secure to put it, so he couldn't get at it again - but yes, I am the mum who let her son eat from the bin, rather than risk a tantrum. 
Or there's the time he got stuck in the cat flap - he was just crawling, and tried to get out into the garden, and got stuck - head, arms and torso outside, the rest inside - and I didn't realise because I had nipped upstairs to the loo. I could hear a baby crying outside, but I knew my baby was inside, so didn't worry - until I went back down and found him.
He survived my parenting, and is now 26, fit, healthy and well adjusted - and your lovely baby will be just the same, I promise. And one day he will be taller than you, and you will get the best cuddles.