The following is culled from my experience. I'm not saying it's fact for everyone, but it was for me.
I've never been married (and when you finish reading this you'll probably understand why
I wouldn't inflict myself on anyone -- especially anyone I cared about ), but I have been -- and still am --depressed. I believe my depression is chronic and it will be something I have to deal with for the rest of my life. In reading your descriptions of your wife's behaviour, I see a lot of things I have done (and still am sometimes prone to) when seriously depressed. It's not a pretty picture and one that's quite embarrassing to look back on when I'm not in such a deep funk because quite frankly when I'm really depressed I'm a major bitch.
When I'm deeply depressed I'm just about everything I despise -- whiny, manipulative, churlish, completely self-centered and selfish, humourless, and almost totally without empathy. It's
all about me and my pain. Nothing and no one else matters. It's all me me me me me all the time. I'm so miserable. No one loves me. I can't do anything right. I'm useless. Wah wah wah. And
everyone has to know it because all the moping around, crying, deep sad sad sighs, and general disinterest in everything.
And of course no one else gets to enjoy anything because the depressed chick makes sure to pull the "oh you go ahead -- I'll just stay here by myself. I'd just spoil your fun" all the while making that super sad, pathetic, whipped dog look which never fails to induce a guilt trip in the other party. If the other person goes out, I get to cry because he/she didn't try hard enough to jolly me into going (even though if I do go I will indeed spoil it for everyone else with my whining and sighing) or doesn't love me enough to stay home with me while I either mope around ignoring him/her, or throw a crying fit because "Now you hate me because I spoiled your night!" wah wah wah me me me. But I really don't care that I spoiled your fun -- why should you get to have fun when I'm miserable? You should be miserable too, and I'm going to make damned sure that you are. And when I feel that way I don't give a flying fuck what anyone else feels.
Can your wife get through this particular funk? Of course, but only if she wants to and is willing to find treatment or treatments that work. And what worked last time might not work so well this time, so she will have to try something else. I don't mean drop the anti-depressants -- especially if they have worked in the past -- but if they don't seem to be doing as much as they should maybe she needs different ones (or different a dosage). This isn't unusual in the treatment of other illnesses -- my mum had chronic heart problems and used many different heart drugs over the decades she required daily medication. When one drug stopped working so well, her doctor switched her to a different one. Why people think if one anti-depressant doesn't work none will is beyond me.
And your support and encouragement can help her, but it should in no way come at the expense of your own mental or physical health. Don't let her destroy you. She has to be the one to decide she wants to get better and then really work on it. You can't
make her get better. You can encourage and support her, but when it comes down to it, it's her decision.