What happened to the glass being half full?
The situation you are in just doesn't seem sustainable in the long term. You are too miserable. First things first, you need to decide if you are staying in the marriage or not. How you will feel, what your options are depend on you having this matter settled.
For what it's worth, I think that once marital misery reaches this level then it's over and all the agonising is over a decision which is fairly inevitable in the long run.
An affair can prolongue a marriage, but it's not a long term solutution. If a marriage is over, affairs can make the ending very messy. I think it all depends on the situation. Sometimes there is such emotional turmoil that it's bound to complicate an already difficult situation. In other situations (where the couple both agree it's over) it may be relatively harmless. The key is in knowing yourself and how you'll react.