It's hard to erase the hurts of the past -- and to be honest, sometimes it simply doesn't happen. Perhaps your letter will encourage other young mothers about to go down the same road you did to stop and reconsider where they're headed.
I do hope you'll do everything you can to let your children know you're sorry for the past, and you hope they'll forgive you for what you did (or failed to do). Don't make excuses or blame others for your shortcomings, and don't express bitterness because they haven't kept in contact with you. Remember the Bible's words: "Reckless words pierce like a sword, but the tongue of the wise brings healing" (Proverbs 12:18).
Instead, be honest and tell your children that you wish you had been the kind of mother they needed. You might also express the hope that they'll learn from your bad example, and be better parents as a result. It probably will be better to put your thoughts in a letter, rather than trying to say everything in a phone call.
The most important thing you can do, however, is to turn to God and seek His forgiveness. He knows all about your failures -- and yet He still loves you, and He sent His Son, Jesus Christ, into the world to die for you. Commit your life to Jesus, and then ask God to give your children a desire to follow Christ also. He can overcome even the greatest gap.