If he is abusive even if you could make him understand how much you struggle he either wouldn't care or would just use it against you to make you feel even worse. The best option would be to get out of that relationship...I don't think its true that having a mental illness bars you from getting custody of your kids I think it would be more about if the mental illness makes you totally unable to care for them.

If you are on disability why do you rely on him for money? I would think you get money from disability and might even qualify for other sorts of assistance...sorry I don't know exactly what resources would be availible where you are but it seems like you need more financial help....and I know there are likely options aside from relying on your abusive boyfriend.