I don't attempt to justify his behaviour but I know when I was depressed, my then husband said I was very hard to live with, and he watched me like a hawk. It is hugely difficult taking on someone else's children when you have some of your own - I have seen friends with terrific issues of who disciplines who, it's not easy.
You need to talk to him. I can't suggest anything else. If you can find a time when it's quiet, and sit down and ask him where he sees your future together, perhaps you might be getting somwhere. I know it's easier said than done, but he doesn't sound very happy either. Only the two of you can say whether the relationship is worth fighting for.
Don't put yourself down though - saying you're not perfect doesn't mean that this is your fault, depression and family differences isn't anyone's fault, and perhaps the situation you find yourself in isn't helping your frame of mind.