I know its easy to see how you've failed in every respect with your spouse. We've all been there, alot of us will take 100% responsibility for the failure of the relationship.
But.... that's a trap.
You didn't do everything wrong. There's 2 of you in that relationship, there's a shared mutual responsibility for the health of the relationship & marriage. If she was a workaholic, if she's always negative, if she's never nice to you, if she never wants to do things with you or share in your hobbies or other good positive things with you, after a while, you're going to stop trying, it's human nature, you can try jamming that square peg into that round hole a million times but after a while you will get the hint, it doesn't fit so stop trying. I get that vibe from you that your marriage went something like that so don't beat yourself up too much.
In fact, turn that frown upside down boy, it's time for you to shine.
If your wife wants to have an affair with another man, I think it's time for you to start considering casual dating, nothing serious and I'm not telling you to go out and hump the first skirt you go out on a date with but you need to give her a taste of her medicine ;-)
Good advice. You are very helpful, Rob. And the upbeat attitude really helps. Any advice on the MC?