There are multiple different phrases that would apply to different parts of the question. No phrase perfectly fits everything mentioned.
"Break free from the past" suggests the person is very actively choosing to change their life, generally in the face of some obstacles preventing them from doing so. This does, to some degree, imply changes that cannot easily be undone.
This would be most fitting to describe an addict moving past their addiction by getting over the obstacles of the physical addiction as well as any negative influences.
This may also refer to "breaking free" from the obstacle of simply being in a rut, or something else that isn't all that extreme.
"Burn bridges" suggests that one makes destructive and generally negative changes (generally by destroying relationships) that would prevent one from returning to how things used to be. This is generally done on purpose, but could also be accidental.
"Cut ties with [the past / everyone I used to know]" refers specifically to ending relationships and cutting off contact, not in a particularly destructive way, but generally in a way one cannot trivially undo.
"Turn over a new leaf" means one is putting most of the past behind you and trying to have a "fresh start" without any elements from your past (or just without the unwanted elements). One could "turn over a new leaf" in a relationship by changing how you interact with the other person, or it may involve moving to a different country, ending relationships, changing jobs, etc. This does not refer simply to making a big change in one's life, but more to changing your perspective or outlook on life (possibly as a result of a big change to one's life).
This does not generally imply that one would be unable to return to how things used to be, but rather that one doesn't want to return.
This seems most fitting for someone moving to a new country to start a new life with no connection with his past, simply because they want to change.
If I wanted to describe the situation in question completely, I might combined two idioms, e.g. "Turn over a new leaf and cut ties with everyone I used to know". Or possibly "Break free from the past to [start a new life / make a fresh start]".