In my experience, it's almost impossible to sincerely apologize to someone or ask them to forgive us, if we aren't also willing to forgive them. And I sense this is the battle you are struggling with right now.
Your letter clearly tells me that you still have great anger and bitterness toward your parents. You probably feel it is justified—and I can understand this, because abusive parents can inflict great emotional damage on their children that they may carry with them the rest of their lives. The Bible's command is clear: "Fathers, do not embitter your children, or they will become discouraged" (Colossians 3:21).
But listen: Have you ever looked at the damage bitterness and anger are causing in your life? The Bible says they are like a deadly poison or corrosive acid, slowly but surely destroying our minds and souls. It says, "See to it ... that no bitter root grows up to cause trouble and defile many" (Hebrews 12:15).
This is why the most important thing you can do is to turn your life over to Jesus Christ. Ask Him today to forgive your past and come into your life. God loves you—and when you open your heart to Christ, His love will begin to change you from within. Then you will have a new desire to forgive others, just as God has forgiven you. God bless you.