the boy and i are over

I've told the boy it's over.
I can't continue with things as they are, I'm just not happy. There's just one thing though.... he's suffering from depression. He's got tablets which haven't been doing much over the past 6 weeks to help him.

He's genuinely unwell right now and he has suffered with this on and off for years, as I have.
Am I cruel to leave him? We've been together 9 months and altho it has been lots of fun on many occasions - I'm feeling selfish and don't want to deal with the situation I've got myself into. But you know what? If people had taken that attitude with me at various times through my bouts - I'd have been utterly broken and utterly alone
What to do? What to do..?