Re: When to throw in the towel?
Your husband sounds immature, I'm sorry to say. If you honestly think that initiating a serious conversation about how he makes you feel would result in him saying "Well you should have thought of that before!", then he's definitely acting like a 15 year old!
The thing is, everyone has different opinions on what's acceptable and unacceptable in a relationship/marriage. ((I'm a bit confused about one thing though: You were flirty and would go for lunch with co-workers in the PAST? Or are you still doing it now?)) Your husband has a problem with you being so friendly with men at work. Granted, he's going about it the wrong way by belitting you with mean comments and threatening to cut you out of the will! JEEZ! The better approach would be to say "Honey, I feel a little insecure when I hear that you had lunch with so-and-so", and have an adult conversation about what can be done to make both of you happy. Picking on you and being insulting is certainly not the answer! You will never be able to come to a compromise if that's his attitude, and for that reason I can't see a reason why you should have to do as he wishes at this point.
Your main problem right now is not his insecurity. It's his awful attitude and his refusal to be a grown up about it! I would keep attempting to help him fix his immaturity problems, but if he refuses to have a serious conversation about it then I can't see how that is even possible.
Have you tried counseling?
There's no need to put up with this sort of behavior, and you deserve to be treated with more respect.