How do I forgive myself, and overcome this self-destructive behaviour :(?

Since you're consciously aware of the problem, the next step is to address your subconscious thoughts that led you there. You probably went down a long path of subconsciously justifying beforehand why you thought it was ok to fail this one cl. Premeditated justifying patterns like that can leak over into other aspects of your life too, and before you know it you'll be lying to yourself subconsciously without even realizing it. Just pay attention to your subliminal thoughts and the subliminal lies you tell yourself...keep a journal if you have to. If you don't want it to happen again, you'll have to figure out how to not let your subconscious premeditate the whole thing in advance.