Adhering to this law of love has always been the core struggle of human nature. We are fallible. We make mistakes, and fall victim to the temptations of pride, and power, and sometimes evil. Even those of us with the best intentions will at times fail to right the wrongs before us.
That is why we are called upon to forgive those who wrong us, as God Himself forgives us our wrongs against Him.
Forgiveness and pennance is the way to continually work things through, yes we may fall, we may falter and fail in our attempts to follow God's Love and love others as God has loved us, but then with our failures we learn, and we get back up and resume our struggle against our failings and continuously ask God for forgiveness for our failings and our wrongs done to him and wrongs done to others.
With Love there is forgiveness, as Love is always wont to forgive.