And everything comes crashing back into yesterday, looking into the past while moving into the future. Perhaps it's a mental illness to be guarded against, alone
It's not a mental illness, but it is a recognized mental/physical condition related to flashbacks and trauma. You are talking about triggers. In counselling trauma and abuse survivors, identifying and avoiding triggers is an important component. I tried to explain this to my mother recently when she wanted me to come for a big family dinner and "forgive and forget" my sanctimonious brother and his bitch wife. I can't be in the same room with them (sometimes even thinking about them is enough) without having the past come crashing back. I've worked hard to become and remain healthy, and part of that is identifying triggers: people, situations, thoughts, that are dangerous to my well-being, and avoiding them.
I think there is also a mourning process we must go through. For some of us leaving represents a loss not only of family and friends, but of our own sense of who we thought we were. It's healthy to grieve that, but moving on means we have to rebuild and replace. I've re-identified myself and become a person I really like now (most of the time). While it's true that a lot of the work I've done was done by myself, I could not have done it completely alone. Supports are essential, whether they be people, things, places, activities, etc. I have had to find something that reverses the effects of the triggers, and that is something unique to each person. For me part of moving on was identifying important losses - education was one, and I think my turning point was going back to university. Writing helped a lot, as did joining several nonprofit boards where I was able to regain a sense of worth through helping others.