OP, firstly, you have to praise yourself for even embarking on study in your position.
Secondly, it's understandable you would be distracted by this man, the attention is probably the only attention you're getting from anywhere. However, he is spoken for, and he's a serial cheat, so you know it's going nowhere.
My DD has just qualified as a healthcare professional. She lives at home and has no commitments, it was HARD. You are a single parent, have a child with a disability, and little support, personally I think you're AMAZING!
I have been a single parent for 10 years+ and that alone is incredibly hard, mine are adults now. Your degree will open your world, so you must focus on the long term. Are your DC of school age? Can you go to the library to study? Have you spoken to your uni? I'm sure they can offer support in some way.
Please don't give up on yourself, your career, your independence, your opportunity to have a better life. Lots of students struggle (even without any commitments) but please ask for help, don't struggle alone.