You didn't necessarily do anything wrong and in fact I suspect you did a lot of things right. Almost nothing is as difficult as being a single parent, and you shouldn't add an undeserved burden of guilt to the burden you already feel over your children's behavior.
You see, ultimately your children not you are responsible for the bad decisions they have made. Yes, we parents influence our children as they grow up, and God wants us to do all we can to point them in the right direction. But even in the best of circumstances, a son or daughter may wander from their parents' teaching and cause them great heartache. The Bible says, "The son will not share the guilt of the father, nor will the father share the guilt of the son" (Ezekiel 18:20).
The most important thing you can do for your children is to pray for them, that God will open their eyes and help them see the danger of their ways. In addition, ask God to give you the wisdom to know when (and what) to speak and when to keep silent. The Bible says, "The tongue of the wise brings healing" (Proverbs 12:18).
In addition, do all you can to keep the door open. If you know you failed them in certain ways, don't be afraid to apologize and ask for their forgiveness. But most of all, urge them to turn to Christ, for only He can forgive them and put them on a new path.