You need to GAL big time. Look closely at what would be the advantages and disadvantages of divorce. As long as he's involved with her be mentally prepared for anything and don't hesitate to hide money if you do suspect divorce may be looming in the future. I would be really angry if my H were spending money on OW (yeah, my H spent family money on her...) but I have my own seperate personal account.
What would you be doing differently if you were divorced and living separately? Will that make your life better? Can you do these same things now being married? Heck, it's easy enough not to wear a wedding ring....
There is no arriving, ever. It is all a continual becoming.