How can people say "Father" to God if they have been tormented or abandoned by their earthly father/their earthly parents?
Human fathers and mothers often distort the image of a kind, fatherly God. Our Father in heaven, however, is not the same as our experiences of human parents. We must purify our image of God from all our own ideas so as to be able to encounter him with unconditional trust. Even individuals who have been raped by their own father can learn to pray the Our Father. Often it is their task in life to allow themselves to experience a love that was cruelly refused them by others but that nevertheless exists in a marvelous way, beyond all human imagining.
How are we changed by the Our Father?
The Our Father allows us to discover joyfully that we are children of one Father. Our common vocation is to praise our Father and to live together as though "of one heart and soul" (Acts 4:32). Because God the Father loves each of his children with the same exclusive love, as though we were the only object of his devotion, we too must get along together in a completely new way: peacefully, full of consideration and love, so that each one can be the awe-inspiring miracle that he actually is in God's sight.
(YOUCAT questions 516-517)
Dig Deeper: CCC section (2787-2791) and other references here.