No parents are perfect, of course, but your children alone are responsible for the decisions they've made, and you should not blame yourself for the path they've chosen to follow. 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).
At the same time, don't give up praying for your children, no matter how old they are. God is able to do what we can't -- and in time they may come to their senses and realize they are on a dead-end road. No doubt the prodigal son's father prayed for him repeatedly every day; he even may have wondered if his prayers were being heard. But in time, his wayward son realized the futility of his rebellious ways, and his father had the joy of welcoming him home (see Luke 15:11-32).
In addition, let them know you still love them, even if you don't approve of the way they are living. They know where you stand; you don't need to keep criticizing them or reminding them of what they should be doing. Remember the Bible's admonition: "A gentle answer turns away wrath, but a harsh word stirs up anger" (Proverbs 15:1).
Seek God's forgiveness for ways you might have failed as parents -- and your children's forgiveness also. Most of all, however, trust your children's future into God's care -- just as you did many years ago.