I know that tiredness first-hand. You are so tired of the fight, the surprises, the worry, the phone calls.
What I can tell you is that there have been so many scriptures and promises that helped me, but God gave me three little sentences over a little time that brought me peace: God has this. He is Faithful. I can trust Him. When circumstances happen, I recite this.
I know you know this, but God loves your prodigal more than you do. He is Creator God and knows the exact thing(s) that will bring him/her to Himself. (Have you ever thought about the different ways Jesus healed the blind? What God did to get Saul/Paul’s attention? What God did to get Moses’ attention? How Jesus rescued Zaccheus? How Jesus went out of His way to meet the Samaritan woman at the well?)
Let me tell you some modern-day examples. God gave a Christian song to the non-musical dad of a musical prodigal; the dad had to ask the son for help; I couldn’t have orchestrated it any better (pun intended)! God also placed a student needing help with writing a paper on Song of Solomon in the lab of a homosexual man whose job it was to read that book of the Bible and the descriptions of God-ordained relationships to help her with her paper! And there are others: a prodigal getting a job with a Christian who led her to the Lord; the removing of ungodly influence from prodigals’ lives, and so many others.
And He has a plan for yours. You are just waiting to catch up in time to His plan that He already sees in reality.
Pray until you get your promise for your prodigal. Then rest in those promises. When circumstances happen, as they will in the lives of prodigals, pray through those circumstances until you get peace, and then rest. Prayerfully rest. (We don’t stop praying.) And thank God for His great love for your prodigal and His perfect plan to bring him/her back to Himself!