believer

Forget about Bob - definitely a red flag that he got cold feet when he heard about your husband finding out. He thought your relationship was all good & fine because there wasn't that true "commitment" involved because you had your family & who knows maybe he has one too? But now that you are talking about leaving your husband - he isn't ready to deal with that.
I agree with one of the above posts to at least give your husband a chance to "hear" you now - in the past maybe the way you were saying you weren't happy wasn't coming across that clearly & he may not have realized how unhappy you really were. I was in your husbands shoes - that apparently my husband was unhappy & felt he had conveyed that message to me but I wasn't "getting it". I finally realized how unhappy he was when I found out about his affair - that made me stand up & take notice. But until then, I had no idea he was "that unhappy". Definitely a lack of communication on both ends.
Wish you good luck in the weeks, months ahead