Leaving behind someone struggling overwhelms me. How do I find some sort of comfort?

I'm in my early 60s and like everyone have no idea when I will die, my mom lived to 74 and my father to 86. I have thought at times I would welcome death, to leave struggles and pain (whether physical or emotional) behind. I have 2 sons in their 40's and neither is financially well off - not because they don't try. Recently my youngest son lost his job and has gone into a profound and horrible depression. Having suffered depression most of my life I know the kind of torment it can be, the fear, the sadness and emptiness and feeling cold no matter how many sweaters you put on, the feeling of being removed from everyone - you can see them but can't touch them somehow. He also suffers from a disorder that could leave him disabled for life and very dependent on others. We have both been struggling lately and both questioned "why hang on" without the other knowing. A crisis of sorts came individually for both of us and we became aware of the others struggles. So we made a promise to each other to not do anything to hasten life's end and I plan on keeping it and I hope he does too. But since then I have seen into his heart and mind and soul as only a mother who loves her child can do for I have known him longer than anyone. I know how loving and sensitive he was as a child. Both of us seem unable to throw off our depression even though we keep our promise. It has been in a very odd sort of way (but nonetheless I wish it hadn't had to be this way) it has been a blessing. I try to appear upbeat for him. I don't feel upbeat at all really but I want to help him feel better and so I can't allow myself to divulge what I think about. My son and I have reconnected so closely again like when he was a child. I don't mean anything weird, I mean his looking to me for guidance and help. He says I'm the only person who really tries to reach him that he can connect with. I am so very glad to be able to give him that. But the thing is I do not see his life getting much better and possibly worse depending on his health and now I have become obsessed almost with the idea that I cannot leave him behind, I don't want to. I can't bear the thought of him feeling alone like he would be feeling now if it were not for me. i do not mean this to sound arrogant, I am no shining example of how to live life best. But I am his mother and I do believe I love him, both my sons, more than even the greatest riches on earth. If riches were the answer to fulfillment then famous people would not kill themselves. But I cannot bear and it terrifies me and overwhelms me with sadness that I know someday I will leave him no matter how much I do not want to abandon him. That is more or less the promise we made each other. I do not know how to deal with this depression which was already pretty bad but now this fear of death because I can't bear the idea of my dearest sweetest child being left without someone he can connect to. Again, I do not mean to sound like I think I'm irreplaceable overall, but I do believe that he and I share so many ways of thinking that I understand him more than anyone else possibly could. I know his past and how it has contributed to who and where he is today. I know he is a loving sweet man who was disappointed in love and suffered many losses as a result but he is a hard worker and now he has discovered that sometimes life isn't fair and hard work does not always pay. And he has told me that he wonders "why try"? I've thought that too. So knowing him so well, connecting on such an intense mother/son level, how can i deal with this? Part of it seems to be that my antidepressant is not working like it should. I had one that worked much better but it made me gain huge sums of weight which I lost when my doctor switched me to something else. It is not an optioin to go back because that's trading one bad thing for another.

I want to know if anyone has similar feelings and fears and how you deal with it? I have always feared abandonment, being separated from my family since I was a child. Religion is not a source of comfort for me in fact it has been quite the opposite due to the things I was taught about it as a child. Religion scares me and is no comfort so what am I supposed to do? I guess take one day at a time and stop projecting into the future? If anyone has any ideas I'd be so grateful to hear?