How to deal with feelings of unbearable guilt and shame?

You must have: Experienced some strong punitive disciplining growing up. Even though you are now grown you regress to feeling like a child caught doing something wrong, even if you have done nothing warranting the feelings of guilt and shame. Consider speaking with a therapist to get beyond this.

Guilt & Shame: Do you tend to feel guilt & shame as a repeated theme in your life or is there something specific that you have or haven't done that has led to these feelings? If the feelings aren't rationale, are excessive or have become a way of life - then you likely could benefit from therapy (l.e. cognitive behavioral therapy). If you need to make amends or to rectify a wrong - then follow your conscience.

Shame: Shame is just about the darkest emotion people can face. It presupposes that there's an audience to our alleged wrongdoings, and that devastating judgement will be passed. Something to remember is that everyone has something they feel ashamed about; it's inescapable. Find a reputable psychotherapist to speak with. Getting it off one's chest in a safe, nonjudgmental place is the first step.