The only sin God cannot forgive is the sin of refusing to turn to Him and accept His forgiveness. To put it another way, the only sin God can't forgive is the sin of unbelief.
Let me use an illustration to show what this means. Suppose you were very poor and someone left you a very large sum of money in their will, and the lawyer wrote you and told you about it. But what if you decided it was all a big joke and you threw his letter in the wastebasket? Would you then be rich? No—not until you believed the gift actually could be yours, and you actually accepted it. The person would have done everything possible to make you wealthy—but your unbelief would keep you poor.
In the same way, God has done everything possible to bring you forgiveness and salvation. He did this by sending His Son into the world to become the final and complete sacrifice for your sins. But what if you refuse to believe in His free gift of forgiveness? The Bible says, "For the wages of sin is death, but the gift of God is eternal life in Christ Jesus our Lord" (Romans 6:23).
By faith turn to Christ and open your heart to Him. Every sin you have ever committed was placed on Him when He died on the cross for you, and He will forgive you completely if you will only receive Him. Don't let another day go by without Christ.