Getting yourself together financially you can do before leaving - at least in terms of sorting out copies of all the information you are going to need for a settlement.
For the rest, I wonder if your heart really is in separation and divorce? Does your desire to find yourself before leaving mask a fear of leaving at all? I remember being in that place. My ex similarly tried to stop me having friends, when we went to counselling he lied to the counsellor which was pretty much a metaphorically huge neon sign flashing ''I don''t care about you or about fixing our marriage''. That was the point at which I decided I had to leave and file for divorce.
So yes I think you are doing things backwards. The decision to leave - whether it involves living separately in the same house, or actually moving out - is the catalyst for beginning the process of becoming independent, of actively going out and seeking new friends, new interests and rediscovering you.
You may feel foreign and alone here, but I''m guessing that after 8 years you would feel equally foreign and even more alone if you returned to the US. Feeling foreign and alone is a state of mind, and many people feel like that even in their native country just because they are a bit different to the rest of the herd.
You mentioned you had to make do without friends which makes me wonder if there has been domestic abuse in your relationship. Psychological abuse is a very clever way of controlling someone and removing all power from them. There are domestic abuse organisations all over the UK who can offer support to adults and children.