I understand how difficult it is to end a relationship, particularly with a young child, when things just go along and nothing serious happens that would clearly justify it.
You could make of list of reasons for and against, or even try counselling (on your own) to help you come to a decision or how to come to terms with a separation.
You are also in a vulnerable position, because you are not well yourself, but that only adds to the awfulness of being in this relationship. Having a nasty person around may make illness much worse, actually.
TBH, I have a lot of support from family, but I dread being ill and having to take care of DS.
However, should the worst happen, there are loads of single parent families where children help to take care of parents. Even young children can be trained to be fairly self sufficient and responsible. And you should be able to ask support from health authorities.
You are strong and can do it. :)