No, God has not written you off–and the worst thing you could do would be to believe that He has. The devil wants you to believe God has given up on you–but it isn't true!
How do I know this? I know it because God has said so in His Word, the Bible–and He cannot lie. No matter who we are or how old we are, or even what we have (or haven't) done, God still loves us and He yearns to welcome us home. The Bible says that God "is patient with you, not wanting anyone to perish, but everyone to come to repentance" (2 Peter 3:9). He also declares, "I have loved you with an everlasting love" (Jeremiah 31:3).
You see, in God's eyes we are all "black sheep"–that is, we have all sinned and rebelled against our Heavenly Father. But God has not rejected us, even if we have rejected Him. He sent His Son into the world to die for our sins and He wants to forgive us and welcome us into His eternal home.
What do you have to do? First, admit that you need God. You need His forgiveness, and you need the new life He alone can give you. Then trust Christ and what He did on the cross for your salvation. By a simple prayer of faith ask Him to come into your life today–and He will.