Maybe it has to do with the fact that the past is the past, and it cannot be changed? So to forgive (either yourself or others) is to let go and gain freedom from something that is impossible to cure? When we fail to forgive ourselves we dwell on things instead of moving forward with a more dharmic attitude. No sense in feeling guilty, just don't do it again, and move on. When we fail to forgive others we dwell on something which again is from the past, and it stops us from moving on while focusing on opening our heart to the world. We live in an age of conditions. We need to learn how to live a life unconditionally.