Staying ?Presumably, you have explored, in great depth, the leaving option and have actually made a fully informed choice to adopt this staying option.
Good for You.
As you have recognised the reality that you are to live in a co-habitation or room mate situation - not a marriage - it would be timely to formalise various aspects of this reality.
At the get go, moving out of the bedroom is the thing to do. As is discarding each and every aspect of thinking about your room mate in spousal terms. You do need to be respectful, you do need to be considerate. You do NOT need to engage on any areas that are unique to a marriage.
Splitting your finances would be a good start, together with splitting expenses. You do not want to be subsidising your room mates lifestyle (nor have them subsidising yours). You need to both need to be financially independent of each other.
If your room mate is reluctant about such a split of finances and expense sharing then you really will have to re-visit that leaving option, as an inequitable financial split means that the room mate scenario will not work any better than the dysfunctional marriage did.
Moving on, your next area is that of your social life. Whereas you may still have mutual friends, it is imperative that you develop your own social network. Sometimes (if it suits you) you may both go out together, but for the most part it will be up to you to run your social life, and much of that will NOT involve your room mate. Nor will you be involved very much in your room mates social life. You will both be running your own social lives, including the odd time where they co-incide and you attend some things together, where it may happen to suit you both.
Implicit in this is that your room mate may choose to root other people (as might you). What you do is none of your room mates business, just as theirs is none of your business. Don't ask, don't tell. What your room mate does is not your concern (as long as they are pulling their weight on expenses) and vice versa.
If there is any reticence on your room mates part to embrace this aspect of the situation, then again, you need to revisit that leaving option again. As with the finances, the room mate situation won't work any better than the dysfunctional marriage did in regard to your social life.
Indeed, it is a fact that even living with a room mate requires levels of co-operation and consideration for others that refusive types do not always have. You may find that your refusive spouse - apart from having been a dud marital partner - is not much chop as a room mate either.
It is actually highly likely that the room mate scenario will prove just as unsatisfactory as the dysfunctional marriage was.
For that reason alone, you still need to know how a divorce would shake out for you (you gathered this information when you considered the leaving option back at the start of this story) and you must have a do-able exit strategy.
There are just as many potential pitfalls in the staying as room mates scenario as there are in the dysfunctional marriage scenario. Any of which could blow up the arrangement at very short notice.
Good luck with your choice of staying.
Tread your own path.