"Depression = Life Sponge" -- well put!
This is an important point. (Thank you, Sanityseeker. Your whole post is insightful.)

I'm the depressed one in the marriage, but I suppose the illness imposes a kind of psychological bigamy on the spouse. The "normal" partner is married to Relatively Less-Depressed and at the same time to Relatively More-Depressed.

If you woke up one morning to find yourself with two wives, a whole lot of adjustment would be necessary. And...you cannot bear the whole burden of adjusting by yourself. As the others here have noted, it may be valuable for you to regard yourself, not as depressed, but as a person with unique needs of your own in this situation.

Please keep on posting as things evolve. You are hardly alone in your predicament.