I'm not sure what to tell you; I was (actually still am) in the position of your wife but my wife left me - separated for 3 yrs - as a result of my depression. I isolated myself & distanced myself from the marriage until she had enough; can't rationally blame her but it's been a complete struggle to deal w/this on my own.
Each situation has to be handled w/extreme care on both sides as a marriage takes effort and commitment on BOTH parties. Perhaps counseling/therapy is needed for her individually & for you (or the both of you) if this hasn't been considered - I know you're not requesting answers but I can relate to both sides of the equation...

I second the idea of couple's counseling...or even counseling for just you to sort out your thoughts. Now that your kids are in college, you two should see if you both really want to make this work as a marriage.

It doesn't sound like you are giving up easily, but there is something to be said for walking away from a hopeless situation, even though it is heartbreaking. I speak from personal experience.

As a 27 year BiPolar w/Severe Suicidal Depression I'm not sure some understand how hard it can be to muster any type of response when your so depressed that just breathing literally hurts. I've been on literally every psych drug there is, been through every type of therapy, CBT, Dynamic, EMDR, BIOFEEDBACK, DBT, ECT, etc. 17 Psychiatrists and well over 20 Psychologist/Therapists. I was married for 26 years, unfortunately my now ex-husband was a "functioning alcoholic", so in the later years even if I felt okay, I rarely wanted anything to do with him. My worst times were in our younger years, I was Diagnosed 1 1/2 after we married. There were times just asking him how his day was exhausted me. But we made it through. In our later years of marriage we had 3 years that were pretty bad, I lived in the master suite of the house, he lived in the family room. I'd come out to make sure the boys had meals and i'd crawl back to my room, onto my bed, away from everyone, all the noise. I just wanted to lay on my bed and pray that if I laid still long enough I'd disappear, because then I wouldn't have to get up the next morning and go to work. See I was the money earner in my household, the college educated one. Keeping insurance on my sons, keeping the mortgage paid, the bills paid, money for vacations...that was all on me. My now ex. would work a couple months here a couple months there...he was a joke. He and his 400 lb new wife are still a couple of jokes.
But when I got home from work each day, met my sons bus, and got to my room at 3:15, that's where I stayed until the next morning. I had no desire to interact with my husband, it was too exhausting. It was hard enough interacting with my 8 yr old. Finally though one day, we got the right medications in the Cocktail and things changed. I had energy, I could get out of bed, I could do things....and I didn't feel like I wanted to die 24/7.

I'll be honest, if your wife is so depressed that she doesn't interact with anyone, getting her to Couples Therapy isn't going to happen. Couples Therapy is not designed for a spouse with serious Depression and the other spouse who's trying to survive that. I know it's confusing that she interacts somewhat with the kids, but it's ingrained into us, we're moms, we have to. Then we have no energy left for anyone left, we have enough energy left to lie on the bed and breathe, I know that's not what you want to hear. Does she do better after ECT? For me ECT saved my life, I had to do 3, 4 week cycles but it saved my life and gave me back my Stability. I was able to return to my Career, get re-involved with my sons lives and get back to living my own....but it's not something easy to go through, 3 days a week, every week. Make sure that you have her write down anything important she can think of before hand, as it does effect memory. I hope that it helps and you get some of your life together back.
If she's seen the same Pdoc for years and there's been no change in her treatment, no getting better even small amounts, it may be time to find a new Pdoc. With all the new meds and treatments out there these days, there should be something that can help her. If nothing does, then you have to decide if you can live your life like that, or do you need to make a change for you.

I'm in a similar situation. Been married 25 years. Happy only a few. Not sure why I never left. I guess I didnt want to disappoint anyone. In recent years he's been diagnosed with bipolar, adhd, narcolepsy, and now severe depression. He lost his job 9 moths ago. He hasnt tried to get a job since. He sleeps all day, and doesnt do anything. He's starting to get nasty with me and I've about had it. I am working 2 jobs (about 60+ hours a week). He just started therapy about 4 weeks ago. I see no changes. I went to his last appt. he had 2 small goals everyday (1) get up by 9am and (2) make a list of 2 tasks and accomplish them). Instead he didnt none of those and cancelled his last appt. I cant kick him out. I cant leave. I feel stuck.

Thanks for all the replies, I see similar experiences in the comments to what I am going through. It really is so difficult feeling like I am stuck, I can't move on because I do care for her and feel that without someone supporting her, her life would completely spiral, she has no outside support and her friends have fallen away through the years. On the other hand, I feel like I am forgoing happiness and fulfillment and worry that I am becoming dragged down by the situation. If I saw a light at the end of the tunnel, or even her expressing rage / dissatisfaction about how things are going for her I would have some hope. If nothing else, reading comments from others going through similar circumstances is assuring.