I wouldn't "snap or disappear" at this point, if I were you.
I don't condone or agree with what your GF is doing... I mean, in my book, it's completely wrong/inappropriate for her to be talking to another guy for "hours", and telling him her "darkest secrets". But, before you jump the gun and severing ties, talk to her... and let her know that her behaviour is hurting you as well as relationship (assuming you haven't already done that?).