Well I think it sounds like what started this trust issue was first the foundation of your ex-W cheating + when you found out that your present W had had sex with her cousin's husband--whom you had already known and established a relationship with and just thought of as family and it would have been unimaginable that something like that had gone on--it triggered this condition.
I think it's difficult enough for some people to have to deal with a significant other's ex even in passing. But to find out that you have been having a close relationship--and that SHE has been having a close relationship right under your nose--even though it is now platonic--with someone she had sex with--must have really pulled the rug out from under you.
It certainly would have affected me and I wouldn't have been able to continuing hanging out, having double dates and spending holidays with them without no longer wanting to do that.
How long had you two been seeing each other, and how long had you known the cousin and her H, when W told you about this past?
