Sending you strength. I think there are different situations. Sometimes as frumpalertt had highlighted, they won't help themselves and put the blame on external things, feigning helplessness and hurting those closest. In that case all you can do is detach with love and, as hard as it is, allow them to fail so they hot rock bottom. Its up to them.
Sometimes however, someone has a breakdown but does want to get better and is willing to do the work but its hard, slow going. Feels like wading through hardening cement. We had someone very close to us have a breakdown amd suicide attempt (not my partner) and they moved in with us.
The difference was they wanted to be better. Not going to lie, it was years. They sought out therapy, took medication. Sometimes they couldnt make decisions so we set out basic things thay were non negotiable. You must go outside for a walk everyday. You must eat a proper meal at least twice a day, these are your chores (very basic) we rely on you to do them because you are still part of the team.
Sometimes they didnt but we didnt let them away with it. Holding the most basic expectations meant they didnt feel completely helpless. We also still talked to them about our problems and they would help, they found that beneficial.
Mental health is a deep dark hole. People can and do climb out of it. You can throw down a rope (helping them get therapy/medication) but its up to them to climb it. The most important thing is you dont get pulled down the hole too. You matter. There is no shame in saying, this is a lot at the moment and stepping away. In fact it may help the person see outside their own tunnel vision.
You cant pour from an empty cup.