I say we get to forgive ourselves because we can. Out of all the belief systems that are out there in the ether trying to lay claim to our souls, there is only one true belief we can have that is real. We are our own master. We cannot offer ourselves the possibility of some eternity since we are not God like, however we are human beings who have full access to our own mastery. We get to make the ground floor decisions about how our life plays out. For me, I don't buy any belief system that only relies on faith that it may exist. I doubt all the time and nothing in my nature as a human being will change my doubt, unless it is evidence.
I understand that I am a biological data processor. Doubt is necessary for me to be able to ascertain the facts and truths of things. It is actually inconsistent with my natural abilities for me to allow a belief system to overrule my need to be curious. If I am not allowed to study and learn about new ideas and methods for discovery, then I am not being the human I was born to be. Which is why I allow myself to forgive myself for doing wrong because I am not one for being out of doing things right. So as my own harshest critic there is little that escapes me when I have left the path of goodness. But when those times do occur I sense and feel my guilt and allow myself the remorse as a reminder to never perpetuate the same actions. Then I forgive myself and move on since it is clearly a human thing to do to be frail and fail. But it is also human to forgive ourselves and then make the rest of our lives better as best we can.