Truth about forgiveness

Forgiveness is very important in our lives. Every day we hurt some one and some other people hurts us. The main thing is that we forgive this people but we never forget the wrong which they did. This is a story of mine.
There was a girl and her mother which any time this girl offends or do wrong to her mother instead of her mother quarreling her she instead tells this girl to go and hit a nail on a particular tree continually. One day this little girl come to realise that there was no longer space on this tree and ask her mother the meaning of what she has been doing and her mother said to her now you have realise what you have been doing and the wrong you have done so she ask her to remove all the nails on the tree after removing the nails she ask the girl to look at the tree and the holes on the tree were still there.
What is this story trying to talk about ??
We forgive people who wrong us but the holes are still there..