If you have a therapist he/she could help you sort this out. I think you need to determine if the problems are coming from you - for example it could be your perception from how you see things due to your depression or the baggage form your upbringing/past relationships. Or they could be from his issues and your issues together.

I think making a list would be helpful - a pros and cons list of staying together vs living alone. For example a partner could be great but if the other partner is deeply depressed, they can't appreciate those good qualities and everything 'seems' awful. I understand how it all seems so confusing. If a partner have bad qualities like substance abuse or they're verbally/physically abusive, then the decision is simple. Wait for when you're not having a bad day and try to make the list realistic rather than emotional