OP, I read your initial post and my heart went out to you. You havve been through and are going through so much and that is SO difficult on your own. In the past I've had depression, like you High Functioning, masking it from everyone while it got worse and worse, and know that crazy conundrum of feeling so lonely and yet being unable to reply to texts.
This might sound a bit pretentious but I'll risk that, in case it is helpful to you. I came across a copy of Aristotle's Nicomachean Ethics and read the section about friendship. It totally changed my attitude to loneliness and to what friendship is and can be. It was years ago so I don;t remember what he actually says but what I took from it is this:
There are different layers of friendship and companionship, and (crucially) all of them are equally valuable to a healthy life.
That's what I'd not understood intuitively. I longed for closeness. Nothing else would do. But Aristotle says embrace all layers.
He says foster work friendships with people you admire for their work ethic or talent etc. Just have a coffee together. Keep it professional but make it warm. Suggest getting lunch together if there's a conference or after a meeting. Or ask a colleague's opinion on a proposal. Or recommend a book or talk related to work.
The next one was really valuable to me. He says, court shallow friendship. It is of immense value. Go out with those mates you can never truly open up to. That light surface friendship can be very sustaining and get you out of your brooding mood, as long as you don't hope it will deepen or try too hard to make it deepen. Lots of people love friendship to stay superficial and any hint of intensity will scare them off. But the friendship can still be pleasant. Go to the cinema or some other easy outing. Meet Up organises this sort of thing if you aren't at a life stage where you have this sort of acquaintance to hand. Maybe you could reply to some of those texts with this in mind - suggest an easy meet up that just takes your mind off things for an hour or so, rather than leading to something deeper. Ironically, when I took this advice to heart and became much less intense about friendship, closer friendships evolved from the shallow ones.
Aristotle also says connect with your community. This is essential to wellbeing. Contribute. Very hard when you feel bad. But at my lowest I helped in a food bank and at his lowest someone i know helped in a soup kitchen. He got life long friends from that and I loved that year at the food bank. It helped me out of depression. So join something communal - local community gardening, church choir, political campaign for a party or cause you support etc. Don't expect this 'friendship' to be anything more than it is on the surface - like minded souls, making their local area a bit better. You may meet great friends. Or you may meet oddball acquaintances. But value this for what it is rather than hoping it will be something else - the coming together of like-minded people for a bit of common good. You belong and you make your mark.
Shared passion friendships can be slow to grow but ime, arethe best. You say you exercise still. That's a good start, Could it be more communal - a running club, dance class etc. It took about two years of yoga before any of us spoke to each other. Now we go on holiday together, out for walks, swap home made jam and plant cuttings.
Join the Ramblers or Meet Up for walks. Arm yourself with a few walking-related questions about best loacl walks, sticks, boots, pub lunch etc to break the ice, so you can avoid small talk about children or partners - I know how grim it can feel to have to say no you don't have any of these when people start talking about family.
Once you feel up to it, try dating. If you go into OLD with zero expectations, but just curiosity about who is out there in the world, then you can laugh about the dodgy dates, maybe make friends with people who you liked but had no spark, and gradually expand your circle.
And meanwhile seek help - loads of it. Swamp yourself in help right now. You deserve it. High functioning depressives are notoriously bad at getting help. You do everything for yourself and it gets harder and harder. Look for group therapy, bereavement groups, online and in person. Call Samaritans, Cruse (bereavement support charity). Sign up for free online therapy with NHS, try some private counsellors or therapists. Ask a local vicar or church outreach person if you can chat about what has happened. Call someone or join a group meet up every day to talk through how you feel. Ditch any shame. Don't think: I'm so lonely I have to stoop to this. Think: these people give up their time to help others and I deserve their help right now. I need support and they are offering. I will take all the help I can get.
Do some woo stuff too maybe - gong baths, 5 rhythms dance, yoga, meditation. These will nurture you a bit and you may find friends there. I've been going to some of this stuff just for fun and it is clear some of the people there started when they were in a dark place and now they go weekly because they love it. Some slightly eccentric things build up warm, friendly communities quite quickly.
You come over as a lovely person who is at her lowest point. I am sure you won't be alone for the next 40 years but it's completely understandable why at this stage it feels like you might be. I admire you for taking care of yourself while you feel this low, for exercising and eating well and taking medication. I think if you keep gently putting feelers out into the world in every direction, without expectation but with curiosity and lots of self compassion, your life will be unrecognisably full and loving within a year or two, It takes time, but I bet 12 months form now you'll see an improvement and 24 months from now, that deep consuming loneliness with be a memory.