Some more facts about new guy might shed light on my jealousy- I'm 45, he's 27, no kids, no responsibilities- C goes over to his place, cooks for him and his roommate dinner, hangs out and watches sports (like we used to)- gets to go out on nice dates, etc.. We really can't afford a babysitter more than once a week, and that is the time that C and I have carved out for our "date night", which has been part of the bargaining over all of this.
C is VERY much like a child, she tends to be bouncy about things, and has emotional ADD at times (when we have "serious" conversations, she will literally change subject mid sentence).
I appreciate the comments and yes, i will keep looking for posts on jealousy. As far as not trusting the guy, part of the challenge is that she won't let me meet him, although I have seen him out a couple of times, and C is so open with me that i feel i do know him well enough to realize he's not a "threat" in the sense of my long term security. Also as part of our current agreement, C has made me promise that I accept and believe that she will never leave me for another man (especially him). Given his background I do believe her, although I wonder what would happen in a future relationship with someone else. An "oh by the way" she has declared to me that she's open to having more than just this one side relationship and continues to meet new men, although nothing sexual seems likely at this time as she committed to new guy that she wouldn't have sex with anyone else (other than me of course)