I have often written that one of life's hardest lessons is that we cannot turn back the clock; when we make a wrong decision or act foolishly, we have to live with the consequences. The Bible says, "Do not be deceived. ... A man reaps what he sows" (Galatians 6:7).
But you are wrong when you say that asking God for forgiveness won't change anything. For one thing, seeking God's forgiveness is the only way to deal with your guilt. Down inside you know you have done wrong; you feel unclean and ashamed for what you've done. But if you turn to Christ and seek His forgiveness, God will take away your burden of guilt. The Bible says, "The blood of Jesus, his Son, purifies us from all sin" (1 John 1:7).
You also need God's forgiveness because you need God -- and right now your sin is keeping you from Him. You need His wisdom to keep from making wrong decisions, and you need His strength to resist temptation and live according to His will. The Bible says, "Blessed are they whose ways are blameless, who walk according to the law of the Lord" (Psalm 119:1).
And the amazing thing is this: God wants to forgive you! He loves you, in spite of the way you've been living, and He wants you to know Him and know the peace He alone can give. Don't turn away from Him any longer, but open your heart to Jesus Christ today.