Sorry you are feeling isolated right now, OP. I think this happens to lots of people at some point in life and it is quite a shock, as often we are not aware that friendships have drifted away until they are already dead.
It happened to me, a while ago, and I rethought what friendship is before setting out to make new friends. This really helped. I decided to fully accept that friendships don't necessarily last forever and that is okay. If they die naturally, let them. Never try to cling on to a friendship that is spent.
I also started to put far more value on shallow friendships and casual acquaintance, without trying to force it to become deeper. That makes it easier to befriend new people. E.g. Go to a fitness class regularly and once everyone is familiar with each other, occasionally go for coffee together afterwards, or Christmas and summer drinks. Nothing more. That's fine. let it be a sociable but casual thing. This can take time. Usually a bout a year of turning up regularly ime.
To make new friends, maybe give yourself three or four new things to do in different spheres of the community. I went to fitness classes, to church, helped at a food bank, and went to an evening class. DH became a school governor, helped campaign with a local political group and joined a choir. Not one of these things ever brought either of us a good friendship. But they did bring us a more interesting life. That meant when we caught up with old friends or met new people through work, we didn't have a flat, isolated energy, and people were keen to stay in touch. We were clearly busy and enjoying life. You might want to do a couple of new things together with your DH and a couple separately - things you really want to do anyway.
I think a good start is: something physical to improve your fitness - dance, yoga, gym, running club, wild swimming etc. Something to do with your local community - food bank or other charity volunteering, community gardening, local political campaigning, serving at church or helping at school etc. Then something creative or uplifting - could be a craft class or choir or am dram, or meditation or adult education evening class.
It's also worth reaching out to old friends from years back. I went to a reunion and realised I missed someone, told her on FB I wished she'd been there and she wrote back. We hadn't seen each other in decades but clicked and have been on holiday twice together now, with plans to meet up again.
Another thing I have found helps is - don't be bland. Reveal the slightly odd side of your personality. People know I'm Christian but I mentioned when chatting to someone I didn't know that well, that I was fascinated by all those pagan rituals that connect us with the turning of the seasons. She invited me along to one. Now I go with her a few times a year. Not religious ceremonies but celebrations of Midsummer or Midwinter. We get on well and now meet for coffee and walks in between quite often.
If I think about the good friends I have seen in the past couple of months - one was a set of people I met online in a shared interest group, another was a set of people who were friends of some of the shared interest group but not online - we met through them, then they drifted apart but I stayed in touch with them too, one was a catch up with old teenage friends, two were school gate mums - neither of them with children in my year but over the years we gradually got to realise we got on. All of these friendships were very slow burners excpet the teenage one, which was 'not seen each other for forty years but picked up exactly where we left off'.
Sorry this is a bit of an essay but I was very lonely at one point and am now happy with the friendships I have. It took a lot of re-setting to get there, and my biggest piece of advice is - it's okay for things to start slowly - never force a friendship. Just get out into the world, do things for their own sake, and friendships shallow and deep, transient and lasting, all worthwhile, will evolve, over time.