“ And is all this just a way for me to use jealousy to prevent further intimacy and vulnerability with her because I'm not sure I'm even stable enough to have a real relationship?”

^This is the big question to answer for yourself. Do you want to have a real relationship with her? If so, can you become stable enough?

The phone number exchange incident is a good conversation starter with her. You can say that you thought a relationship was progressing with her and felt confused and a bit jealous when she gave him her number right in front of you. This conversation will define your relationship and she’ll tell you how she feels.

Just my opinion: I’m traditional in dating do’s and dont’s. If I were out with a guy that I kiss, I would not be giving my number to another right in front of him, unless it was legit business or long-lost cousin.