11pud I am so sorry, this is so hard.
Please take a step back, you have a brand new baby, a premature one (any other kids?), sounds like your dad's health is bad, your brother has just divorced and you might even have a touch of Post natal depression (IMHO - just possibly as it is very common I think).
In your shoes I would ask to speak to someone and get some help to get your thoughts in order.
It is totally fine for you not to want to be with your partner anymore, he sounds like he is putting a massive stress on your relationship at a very critical time for you both.
But before you leave I would get some expert advice, both for the emotional (and dare I say medical if you do have any sort of post natal baby blues) and for the practical/financial side.
I think you need to explain to your partner what has happened and what his part in it has been.
If he closed the business, or ran it as a part-time hobby and got a full time job, and stopped spending money he does not have, would you want to stay? If so, give him the option.
If not, then fine, go.
At this very important and difficult time (when you should be happy and resting) your partner has put you under massive strain, he needs to know this, whether you choose to give him the option to change or not.
Even if you do leave he may well want a relationship with his child/ren and he is financially partially responsible so either way he will need to step up.
Please get help, Relate, mediation, financial advice if you can find it for free locally, and prepare yourself for the future. You are right to want to leave but either way your dh is going to need to sort this out!
I think your dad could be a support here, whether you live with him or not, hopefully this will not put an undue stress on his heart but it's your call whether to involve him or not.
IGNORE anything I have said which is unhelpful. 
Good luck 