@st Colin, hi mate. Sorry to hear you suffer with depression, but if I may offer some advice. I used to race (6yrs ago now) and have always been fit, but I hated racing, and looking back now I can see why. It ruined why I started mtb ing in the first place, it wasn’t social and very elitist.
The advice from the other posts is great, concentrate on just riding, getting out and having fun. See if you can get involved in club/pub rides. After a ride look at what went well, try and focus on the positives ie. Wow, I’ve never managed to ride up that section before….
Steve