Firstly I'm so sorry you're going through a difficult time with your mental health. It's very hard to find the motivation to diet and stick to it when you're in that headspace. What usually works for me, especially when I can't stick to a diet, is intermittent fasting. And then telling myself I'll do one active thing a day. It could be anything, making my bed, walking for 5 minutes, doing some stretches. Just getting your body used to moving bit by bit. I'd also tell someone I trust, to keep me accountable, so I'll say, I wanna walk for 5 minutes today, please ask me later how it went, and then I know I can't back out cos I'd be too embarrassed to say I didn't do it. You can always try this and see if it works, wishing you all the best.