Oh Beth, it is an awful feeling when you suspect your H of cheating. I think most of us here understand that sinking feeling. I was sick to my stomach for months, and ended up losing 30 lbs because I couldn't eat. Just vent here, and try and keep it to yourself (says she who prefers to confront, and always does, but it works for me) and see where it leads. Patience really is a virtue in this sitch. A painful virtue, however.
On ultimatums .... never give them unless you are willing to accept the consequences and follow through. For instance, I've told my H that I want the truth, and if I catch him out in another lie, I will only speak to him where our co-parenting R is concerned, and that I will not want to acknowledge him any other way. My reasoning is, how can one have a conversation or discussion with anyone where one can only expect lies - doesn't make sense. I have truly had it! And, I am willing to face the consequences of my ultimatum.
Good luck!
Me:57 H:52 M:28 Got another lawyer last year and filed. D35,S/D twins28,D22 EA4/04 End? Who knows? "Life is like a mirror. Smile at it and it smiles back at you." — Peace Pilgrim