Ok ignoring the fact he is younger and unattached. You need to find strength in your long term relationship. Every new secondary relationship kind of feels like this. The secondarys get the icing. The fun happy go lucky un-attached feeling happens with someone new. You need to find your relationship strength in the struggles you have had as a couple in building your long term love
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.
This is a double edged sword. You can't confront the other side, so you don't know what or who your opponent is. It creates a level of unknown which can create massive insecurities.
Another massive insecurity is the fear of the unknown. While we haven't read her side of the story, it doesn't appear she is giving any lee way to you. She is pushing constantly. At some point she needs to slow down or even in your very acceptable behavior you will probably break.