I was in a similar situation. I had a friend I was trying to witness to and hoped to see saved. He came to church a few times with me and there was one Sunday where I knew in my heart God would save him. A salvation message was preached and mu friend was severely convicted. However as we were talking he did not accept Christ. I fell apart.

He and I had made some compromises before but after that day, I became very angry at God. It wasn't His faulty friend had rejected him and I'm sure that was the day He intended to save him....sadly he has a free will though.

Later, my friend made advances and I entered into a relationship with him. I had been in love with him for years
However I did not live godly. After a year and a half we had a huge falling out.

I have regretted my decisions and I've sent him countless material to witness to him. Also, I resolved tp.pray for him every day. I have hoped for reconciliation, but it was delayed. Last year he contacted me out of the blue. However he failed to mention he not only had a girlfriend but that he was moving in with her.

I had to cut off contact but I sent a letter and one last book. I explained to him my concern for his soul and that I thought he had a wrong view of the Lord due to my poor example. However, I also pointed out that he needed to make serious changes before we could talk again.

I don't know if the Lord wants to bring him back or not but I do believe He led me to pray for him and witness as far as I could. I hope in time to see him again and that he will be saved.

Don't know if that helps or not. I just saw myself in your story. I often thought of writing a book to cope, a work of fiction or something, but never did. My pain proved to be a road block. Hope this will encourage you and show you the right direction.