One reason I wanted to reprint your letter is because it is a sad reminder of what can happen in the wake of a divorce. Many people today see divorce as an easy way out of their problems–but in reality, the results are often far different. Perhaps your letter will encourage someone to stay with their marriage and work out their problems, with God's help.
Is any relationship with your children possible? Perhaps not–but you won't really know until you try to reach out to them. And that's what I encourage you to do, gently and one step at a time. Take time to write a brief note to each of them, telling them you are grateful you could see them last weekend.
In addition, tell them you know you have failed them as a father (if you can honestly admit this), and that you regret this. Don't try to defend what you did in the past or blame anyone else for what happened; that will only stir up resentment. The Bible says, "A gentle answer turns away wrath, but a harsh word stirs up anger" (Proverbs 15:1).
Yes, it would be wonderful if you could be reconciled to your children someday, and I pray you will. But your greatest need is to be reconciled to God, for only He can cleanse our past and give us hope for the future. Open your heart and life to Christ today.