Feeling low

Jordan
33 weeks and 3 days with fraternal twins. My first born son is only 18 months.
My partner is lovely in many ways but understanding and sympathetic aren't words id use to describe him.
I feel like I'm alone in this pregnancy a lot of the time and that because I have no family I'm even more vulnerable. 
The idea of being a mum to 3 and struggling and my son suffering because of it really gets me down. 
I just don't think I'm cut out for this. The closer it gets to my twins arrival the worse I feel. I've lost interest in food, in going anywhere or doing anything. I'm miserable a lot of the time. And not just pregnancy related suffering kind of miserable.
What's wrong with me?