"I used to have a sense of humour. I used to not be so selfish and focused on my own problems. I used to have a life that wasn't this one."
And you will have one again, and I hope it will be much sooner than you think. Your husband sounds like a very selfish inconsiderate man , but many men are immature and unable to be supportive. Do you still love him and want to be with him? It sounds to me as if he could be a mummys boy and the MIL appears to be reluctant to let go and would love to tie him into dependence upon her by getting hold of the children.
You will ruin your life for a lot longer if you allow this to happen.
Can your own family help out, your mum or a sister? Have you told your mum how desperate you feel? IF not then you need some outsider help , even if its just a cleaner/housekeeper to come in and sort the house and washing and ironing for you. Another possibility to consider is putting your baby daughter in a day nursery for a couple of days, or even just a couple of mornings a week, some will take very young babies. This may not seem 'ideal' but it will do her no harm and much much better than her losing her mum altogether and having to be raised by her granny!
"I never thought we'd have dd. I feel like she was never meant to be and like she will be snatched away at any second. The utter horror of waiting for her to die all the time is just too much."
Now you know this is irrational. Your daughter is healthy and its your own illness causing you to worry so. Take your daughter to see your GP and ask him to check her over. Tell him about your irrational fears. Maybe simply being told that she is normal and healthy will give you an anchor to return to when these scary thoughts creep up on you.
Have you got mum friends? This is really important when you have a baby. If not then do join a local group, churches often run them. If you tell the organiser how depressed you have been feeling then I am sure they will push you ahead on any waiting list.
Don't give up on yourself, your children love you and need you more than anyone else in the world, get the practical help you need to enable you to be the person you used to be and want to be again. Things will change, nothing goes on for ever.