you aren't going crazy, this is a common occurrence you are going through. Have a look at some threads on jealousy. You can find them by doing a tag search. Its all very normal and takes time to adjust to. It takes work to work out what you are going to do when she is out, find what boundaries you have on time with her and to work on a metamour relationship with him. Yes, I would get to know him better. It can only help.
Anyone want to be friends on Facebook?
Send me your name via PM