There used to be a posted in my Weight Watcher meeting that said, "Behind every action is a positive intention." I loved that poster, and it's something that has really stuck with me and made a huge difference in my weight loss journey (and outside of it).
That poster sparked heated discussions in meetings -- there were people who didn't believe that there was any positive intention to some actions. To me, however, that quotation opened new doors -- even the worst behavior has some positive intention (attention, deflection, affermation of an idea, comfort in the familiar)... and nailing down that intention can be the key that opens the door to intentional change.
When I want candy or ice cream in the evening, it's likely because I feel like I deserve a reward (for something I did well) or a consolation (for enduring a bad day/situation)... and recognizing that lets me choose how to meet that need in healthier ways. If I just denied myself the sweet without meeting the underlying need, I would not be successful in the long run, because the need would get stronger and ultimately derail me. If I can find another way to reward/console myself, I can meet that need and destroy (rather than feed) the craving.
When I find that I'm beating myself up, it's because I want to do better. So, instead of allowing that soul-crushing negativity to continue, it's better for me to think about what postive intention my "bad" action was trying to meet... honestly digging into those intentions is the only way to move forward freely and lightly, forgiving myself and learning and growing into the person I know I can become.