Pretty much any reasonable chance of success is predicated on successfully treating her illness. Ultimately, she needs to know that either she accepts treatment or you are going to leave. You don't have to be harsh or phrase it that way, but things are very unlikely to get better so long as there are relationship problems caused by an untreated mental illness.
Even with treatment, things would not be easy, but at least there would be a good chance that things might get better.
Considering what I have read on here, I think it was somewhat a blessing in disguise that things fell apart before we could get married. These relationships are really tough in the best of circumstances and invariably fall apart in the worst of circumstances (when the sick partner refuses to consider treatment).