"Can't God just forgive?"
Imagine a criminal says to a judge in a court of law - "I have broken 10 serious laws, but can't you just forgive me and let me go?" A good judge will say "No way, because the law has been broken, there must be punishment."
But let's say a judge decides one day to say to himself, "I'm feeling really forgiving today, so no matter what criminal stands before me I am just going to them go free with no punishment." Is that a good judge or bad judge? Obviously, a bad judge.
Now, God is not a bad judge and therefore cannot just forgive. Even if you say sorry and tell him you won't do it again, justice still must be served for the violations of his law that you've committed.
That means since we've all broken his laws (lying, stealing, swearing, lusting, etc), we deserve hell. So the only way God's justice can be satisfied and yet he still show mercy to us is if a perfect person pays our fine for us. And that is what Jesus did on the cross 2000 years ago for whoever would trust that Jesus has died for his sins. When will you do that?