As a person who has been hospitalized for severe depression, OCD, and continues to suffer from PTSD, I feel as though you were speaking all the things that I have felt. What saves me is Buddhism in the way that it was taught by Thich Nhat Hanh. Also, the works of Pema Chodron have helped me greatly. You are looking for clarity. I got that from reading WHEN THINGS FALL APART. Some people find her message scary, for she lets us know that none of us is guaranteed anything, no matter how good we have been, no matter how many things we have "done right". But rather, an acceptance of the reality that" things fall apart, then come together, then fall apart". The only way to find purpose is to live in the "now", which Thich Nhat Hanh taught using mindfulness. Of course, sometimes we are so temporarily broken (I have been there many times) that we really need to reach out for good professional help and maybe even some medication. Medication is sometimes necessary to allow sleep, or some calming of anxiety, in order to allow to get the brain to a place where it is not stuck in the amygdala fight/flight/freeze stage.