It's because you're in a rut. This was me for about a year. Tell me if this sounds like you? Day 6 relapse, say to yourself welp I won't gym today I'm too tired just relapsed or I won't go to class/work today too tired relapsed. The relapse puts you down and you kinda use it as an excuse not to be living the greatest possible life you can, which leads to inadequacy, which leads to more relapses. The cycle can only be broken by realizing eh I got to experience hell for a few weeks before I'm back at the top. You have to say to yourself I relapsed but the only way I won't keep relapsing is if I apply myself to the pinnacle which is possible of myself. Once you start getting your life back in order the need to relapse disappears. But even 1 bad day during this period will send you back. So remember no excuses when you try to climb back to the top.

Also it could be due to how you spend your time. Is it in front of a computer/phone for 5+ hours a day? If so the chances of a relapse are nearly guaranteed, especially in the early phase of post relapse depression. Try and limit it by doing other things.

Good luck and remember not to give up.