I agree that just facing your fears is gonna be what's best here. Good on you for working on it. Does it not help at all to think about the fact that the time you spend with your gf doesn't make you love your husband any less?
There are some resources here that may be of help to you: http://www.morethantwo.com/becomingsecure.html
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.