I'm sure you probably did the best you knew how to do under the circumstances, and you shouldn't bear the weight of responsibility for everything that's happened with your son. He bears the responsibility for his decisions, and his father, too, probably had a part in shaping him.
At the same time, if you know ways in which you did fail, bring them to Christ for forgiveness and then leave them there. In other words, don't carry your burden of guilt any longer for God doesn't want you to. God loves you, and Christ came to lift sin's burden from our shoulders by His death on the cross for us. The Bible says, "Cast your cares on the Lord and he will sustain you; he will never let the righteous fall" (Psalm 55:22).
Pray for your son; even if you don't know where he is, God knows, and God is able to bring him to his senses. No, I can't promise he will return someday but it's far better to pray for him than to ignore him.
Then put your life and your future into God's hands, and thank Him that His Son will never abandon you. Thank him also for your other son, and ask Him to help you keep a healthy relationship with him. Divorce can have terrible consequences, as you have discovered but God wants to mend our broken lives, and I pray this may be your experience during this Christmas season.