I'd agree with Dons_1988, it seems like you know what's eating you, and pretty much how to fix it but just can't get the bridging point.
I don’t expect all the answers or a solution but I do know that I have to change, to stop being angry, to properly confront and sort out work issues and maybe try some new clubs or events but it just seems like a closed loop of hopelessness.
This, and the bit about joining the new gym, seem like you're putting too much pressure on yourself, running a million scenarios thru your head and trying to plan ahead for them happening which is leading to anxiety and paralysis. You've mentioned hillwalking - I remember when I was a kid and my old man would take me into Tiso's in Glasgow, they used to have a noticeboard up for group hikes and the like. If those are still things - might not be in this day and age when everything's online - would be worth taking down a couple of numbers/emails. Don't commit to anything too much, but have it there. Alternatively, join something like the Ramblers Association - if you're putting member subs in, it might give you that bit of motivation to go and get a return on the money. Most of the events will be a haul around Arthur's Seat or similar rather than a full blown ice climb up north, so should be fairly casual and pressure-free. I can never beat going cycling to get out of my own head when that needs to happen.