I understand, too. I left work earlier than planned, and whereas I do some part-time and voluntary work it is all quite isolated, in contrast to going into the office most days and being around other people.
My children have properly moved away now. I didn't really count when they were at university, or for a while after, but they are both now settled in the towns where they studied (or nearby - my son moves to a new house on Wednesday), and unless life takes an unexpected turn I don't think they will come back to live with us.
It's difficult to feel useful sometimes, but I'm quite happy to take each day as it comes and see what happens. I did make a conscious decision to meet new people after Covid - I'd worked quite a distance from home, and wanted to have more to do in my local area. That's worked quite well. We go away a lot to our caravan, which gives a change of scenery. I do enjoy it, but sometimes it feels like it's more difficult to be part of things at home, as I have to ration regular commitments or try to group them together, so there's not too much coming and going.
I got rid of a lot of large cooking pots recently, as apart from Christmas I don't have large gatherings round the table any more. That was hard to do, but not seeing them every time I reach for a 'serves two' dish is better. I have gradually made the house more 'ours' in other ways, too. It isn't set up as a family home any more, but it works better for us as a (mostly) retired couple, and could change back again if the need arose.
I don't know what you did for your work, OP, but could you do something voluntary that would make you feel useful? It doesn't have to involve going somewhere if your health isn't up to it. You can do lots of things digitally now, if your skillset allows.