It's like my head is stuck in the wrong gear somehow, but there's no real reason for it.
I think you feel exactly as most humans would if they sat at a desk all day doing something they found not particularly meaningful, and then sat on a sofa all evening doomscrolling. There's a good reason for it, you've just got to take steps in the direction you want to go, and keep going. Not try to snap out of it or fix it overnight.
How you feel is how most people would feel, there's absolutely nothing wrong with you, but your human isn't getting what it needs. You need to see some faces, and feel some connection to place, and get some purpose.
I would start with joining one group that takes your fancy, make it a weekly thing, one evening, and make yourself go, even if you hate it, for at least six weeks. It will give you something to think about and talk about and get the wheels moving.
Spend the money you have available for therapy on physio and get that tendonitis behind you. Then, get outside for at least half an hour a day. Even in winter. Get yourself a decent waterproof, keep it by the front door, stick your boots on and get outside. Doesn't have to be a run, it can be a slow shuffle round the block until you can do more but you'll notice things changing in the world.
You need to move your body, if you are behaving like a sick person and not moving, then your brain can't tell the difference. Swimming, or weight training, until the ankle is fixed.
You can't shut yourself in a house on your own for five days and nights every week and expect to feel anything other than lonely and a bit sad. It's almost like a self imposed prison! You need to kick yourself up the arse, do something, anything, as a start and things will naturally start to improve.