hello love, difficult time for you, I get your dilemma re the capability thing. I worked as a support worker, also for a charity, and although I had been with the company for several years, I hadn't disclosed my own mh issues. I became so unwell and afraid I would put a person who I supported at risk, I resigned, in order to deal with my issues and hopefully recover. I couldn't go to my line manager with these concerns for fear of getting in to trouble for not disclosing in the first place.
looking back now, I would have liked to have talked things through honestly and openly with the company, and took their guidance on my post/situation.
as we know, mh affects us all very differently, so I cannot advise you, its so difficult to make these decisions/calls when were not feeling 100 percent ourselves.
I don't know if the decision I made was the correct one, but I was exhausted and in my darkest moments I feel weak and a failure for quitting, as I loved my job and miss it loads.