I don't see much clarity on what you want Grace. It's clear from his behavior and number of affairs, that you need to take strong action. If he doesn't see a reason to change, it doesn't seem that he will. What is your goal in talking about the separation?
You drop the expectations of him doing the right thing, certainly don't look for reciprocation. But his moping around, being indecisive isn't good for you or the marriage. Start getting a life, starting working on yourself. Let's see if he doesn't perk up, and if he doesn't then oh well, your life is all the better.
Don't focus on what you can't control, like his depression causing this or him moping, make your life and marriage better in the ways that you can.
H 34 W 29 BD 3/12/18 Divorce Busted Spring 19
It is not things that bother us, but the stories we tell ourselves about things.