When not in front of her, cry 100% of the time if you have to. In front of her, you are an alpha male. YOU ARE TOO GOOD FOR HER! That is how you should feel, and conduct yourself. There is nothing respectful or attractive about a crying, weepy guy. So in front of her you are a rock. You should be taking the attitude that "you are leaving me? YOUR LOSS!" It is hard to cry with that perspective.
Get angry if you have to (internally, not outwardly). Have an attitude of how dare you blame me for your mess! Use that to keep from being the sad, "woe is me" guy.
Steve, you got this. Dudes with our name are strong. Independent. We don't need a woman to know our worth! So find your worth internally. You are awesome, whether she stays or goes! BE AWESOME!
I definitely don't feel that way right now, but I know you're right. I feel like a failure.