Hey JudgeNot,
I'm sorry to hear what you and your wife are going through, I will be praying for you both and your daughter!
I would say my dad probably went through much the same thing with me...
I have grown up going to church, surrounded by a strong christian family.... I was 17 when I got into the wrong crowd, I was dating a non-christian, going to parties and getting drunk. Even smoking marijuana a bit too... I wasn't going to church and I didn't keep much in contact with my parents.
When my bf and I broke up, I was heartbroken and I ended up turning back to God... I wanted that happiness and peace back that only God could give me.
I do believe in the power of prayer and I believe that your daughter will eventually come back to God. Sometimes as teenages we have to go through certain things to learn from our mistakes, and to never go back there later in life.
Have faith in Jesus and expect your prayers to be answered!
Don't think that you failed as a father because at 18 your daughter is old enough to make her own choices. All you can do is pray for her and be there when she needs you and when she comes back to God.
God Bless