It's a hard lesson to learn (and some people never do learn it, I'm afraid), but you can't change the past. What's done (either for good or for bad) can never be erased, and the consequences will remain with us the rest of our lives. Perhaps your letter will help someone avoid going down the path you once chose.
But you can change the future, with God's help. And He wants to help you, because He loves you and cares what happens to you. So far in life, you haven't paid much attention to Him, but why should that be true in the future? Instead, put your feet on a new path by asking Christ to come into your life, to forgive the past and help you in the future.
How will God help you? I can't promise you'll be able to overcome all the hurts your children feel. But you can reach out to them, telling them (perhaps in a letter) that you're sorry for what happened, and you have now given your life to Jesus.
In addition, Christ will give you hope as you grow older. You'll know He is always with you, and most of all you'll know that someday you will go to be with Him in heaven. Because of Christ's death and resurrection we have "the hope of eternal life, which God, who does not lie, promised before the beginning of time" (Titus 1:2).