A.:
First - Ask yourself this - am I better off with or without him? If the answer is with - then YOU need to make a decision on HOW YOU want to handle it. If it is without - then you need to kick his butt to the curb. PERIOD.
Second - his not keeping is dick in his pants is NOT YOUR FAULT. To have him push this off on you is TOTALLY unacceptable. he's an adult - band member or not - he's in control of his actions. No one is holding a gun to his head to make him text her, etc.
Third - DO NOT MAKE HIM CHOSE. He's an adult. This is YOUR life too. You TELL HIM what YOU are going to do. If it's the way he's going - then great - if not - his loss, not yours.
Fourth - the kids? REALLY?! Do NOT use the kids as a pawn. THEY ARE NOT! The kids are better off with two parents who love each other and respect each other. They can and WILL pick up on the nuances of comments or the body language you two are exhibiting. Most kids would rather have their parents happy than fighting.
My ex-husband cheated on me - not once but several times. It wasn't me - he was getting sex at home - so he couldn't say "aww pity poor me" he just couldn't help himself. I forgave him the first time - but when it became habit and he wouldn't wear a condom (I could've begged him to stop - but really - he was an adult free to make his decisions) I told him he wasn't worth my life. No, I don't condone cheating - but I'm not going to beg and plead.
Quitting the band will solve nothing. He needs to grow up and realize that he is married. Unless you guys have an open marriage - which it doesn't sound like you do - you need to make a choice. DO NOT MAKE HIM CHOSE. He needs to grow up.
YOU ARE NOT BEING SELFISH. YOU ARE MARRIED!!! YOU WANTED A SINGLE PARTNER!!!! He NEEDS to grow up and figure out WHY he feels the need to communicate and/or fornicate with this other W.. Hell, I'm bold - I'd call her myself and ask her what her take is. Does she know that he says that their "relationship" is "NOTHING"? Oh yeah - I've confronted other women. He may be telling her one thing and you another - funny what you learn when talk to the other party.
HE MUST GROW UP!! DO NOT LET HIM CONTROL YOU!!! This is HIS mess, NOT YOURS!!! DO NOT LET HIM USE GUILT OR THE KIDS TO HAVE HIS CAKE AND EAT IT TOO!!!!
You can make demands of him - you can drop him off at the bar for the concert - but make sure he has enough money for a cab home and tell him YOU EXPECT HIM HOME. No, you aren't his mother, but you DO expect him home. I'd like to know where he is staying on the nights he doesn't come home. If he can get to a hotel or a friends house - he can get home. That's my take.
DO NOT EVER let a man manipulate you, guilt you or use the kids as pawns. YOU ARE IN CONTROL OF YOUR LIFE!!! NOT HIM!!
Please feel free to inbox me personally. YOU ARE NOT ALONE!!!