I feel like a husk of my former self

2 years ago I felt like I had the world in my hands. Many friends, great relationship, numerous academic achievements, top physicality. I was basically feeling like a god.

Then it somehow went wrong. Maybe I failed to set appropriate boundaries with my partner. Maybe I stopped being such a pushover, an agreeable option, around my friends. Maybe I gave up on my body and health, riding on the "high" from my recent achievements. Maybe I did all of the above wrong, or something that does not cross my mind right now. But currently I am in the same relationship, having withered the storm on many things (which certainly took their toll on us), friends I have disappointed, alienated or even hurt, feeling alone, depressed and trapped.

I do not know what to do, how to cope with the situation. Has anyone experienced something similar? If yes how do you cope with it? Any advice is welcome.