Only if they are guilty do they have any reason to fear punishment. If they are not guilty, they can be reassured in knowing that they do not deserve punishment, and therefore will not believe in punishment, and therefore will stop scaring themselves.
The person is not afraid because of what is going to happen to them, they are afraid because they BELIEVE that it is going to happen to them, because they believe they deserve it. Having believing they are guilty, they actually believe in their own deserving of punishment, and so they KNOW it is going to come, because they themselves are going to MAKE it come, and they therefore see the punishment as inevitable. They cannot escape their OWN belief in self punishment, and so are frightened by their very own will and determination to harm themselves!
They are really afraid of their own punishment. This must be undone at the cause which is guilt, and not the effect, which is a projected cause of fear.
Beyond this, if someone is feeling guilty, you can't reassure them that they're not guilty either. Instead, you need to reassure them that they did not really sin at all. Sin produces guilt and the belief in sinfulness makes a person feel guilty. Undo the belief that they are sinful and there can be no need for guilt. Only the sinful will feel guilty.
If the person believes they sinned, they will attack themselves and condemn themselves with guilt. The condemnation is coming from themselves, not from anyone else. Even if it seems like shame or misplaced causes for guilt, the decision to be guilty is their own, based on their own belief in their own sinfulness. They themselves WANT to find themselves guilty, and so cannot help but feel that way, because they don't want to undo the belief that they sinned at all. Sin is not real. Nobody sinned. Hey presto, no guilt.