“How do I know that God has truly forgiven me?”
You can be confident that God has truly forgiven you because God says so. God is trustworthy. Therefore, what He declares and promises is also true. God says that He will completely forgive you if you trust Jesus to be your Lord and Savior.
“I feel something different inside me. Is this the Holy Spirit?”
God promises that the Holy Spirit will dwell within you and gives you new life from the moment you surrender your life to God and make Jesus Christ your Lord and Savior. If you feel something different, that may be the effect of your new spiritual birth.
“I don’t feel any change or anything special. Did it work?”
Becoming a Christian is primarily a matter of your will (decision). “It works” so long as your decision is genuine to make Jesus Christ your Lord and Savior.
“Will God really stay with me forever, even if I make mistakes again?”
God will stay with you forever because you, as a Christian, are now in a new relationship with God your Heavenly Father. When children disobey their father or do wrong things, his relationship with them never changes. He remains their father. Now that Jesus is your Lord, God will always be your Heavenly Father.
“I have done some terrible things in the past. Are you sure God forgives everything?”
Jesus died on the cross to pay the penalty for ALL your sins — no matter what they are.
God’s forgiveness means that your relationship with God has been restored and that you are saved from the eternal separation from God (hell). However, you may still face the consequences of your sins. They are not God’s judgment.