One of the biggest things I've had to deal with with chronic illness is the feeling that I've slid back behind square one, and I'll never be able to get there again.... I'm grateful its just a feeling.
Back at square one can be at a lot of different levels for a lot of different reasons. Health, job/career, emotional, relationships, friendships, and so on. But when one finds themselves "back there" one actually now has a wealth of information that can be used to move forward again: What worked, what didn't, and what to do different.
Most of the time I am the one that put myself at square one again. Sometimes it was on purpose because the route I was taking wasn't benefitial... i.e. my alcoholism was taking me the wrong direction in life, even though I was a fully functional and successful person while my alcohol was developing, right up until the end of it when I HAD to do something about it.
Moving to a new city will put a person at square one when it comes to employment, knowledge of the area, and friendships/relationships.
For me throughout my life and career, I just went with the flow of things. I seemed to end up having made mostly good decisions along the way. Relationships on the otherhand .... Well that part of me was busted for a long time.