Absolutely plan this out very carefully because as soon as your A discovers that path you are contemplating he will obstruct it with all of his power to manipulate the situation... this is pretty universal with A's.
If you carefully think through every potential scenario and have already mapped out a solution to that situation you will be so much less stressed during what will be a difficult and turbulent time when you do seperate.
Legal advice is a great idea as well as a counselor... remember... it does not have to be sudden unless there is danger. Take your time as acceptance and thinking it through will help you with your resolve and firmness when he finds out and starts running interference...
Support and heartfelt prayers your way... remember that path to peace and happiness goes through the forest first but you can do this!