Some people have serious mental health conditions, which are not going to get better - they may come in waves; but the fundamental conditions are always there.
My brother had an episode of paranoid schizophrenia; but he never went back to his previous normal self. He may not have the positive symptoms like hallucinations, but he still has negative ones.
IMO, it’s ludicrous to talk about some people with schizophrenia, personality disorders, bi polar disorder, OCD, etc as if all they need is to want to get better! I am sure every person with enduring MH problems wishes, they were better - but wanting is not going to cure them, any more than it would cure physical health conditions like MS, Parkinson’s, cancer, etc!
Another problem is getting effective treatments anyway - the waiting list round us for psychotherapy is 2 - 3 years. Then if it doesn’t work, and it turns out, you have complex PTSD; they say you need a period (6 months) of “consolidation” of the psychotherapy, before they will put you on the waiting list for EMDR….If you are suicidal, it can be very difficult to get private treatment, because it’s so risky for the therapist/psychologist/whoever. They push you back to the NHS.
Practically nobody wants to be ill! Why would they?