When you feel guilty is it conviction from God or is it accusations from Satan? When you have sinned God will convict you of that sin. But if you ask the Lord to forgive you and you still feel guilty it is Satan accusing you. God promises to forgive you and that your sin is forgiven and forgotten by Him. So if you are still feeling guilty after you have asked forgiveness it is Satan making you feel bad about yourself.
Of course, if you have hurt someone you need to ask their forgiveness too.
I found a blog post about this that is very helpful. If you would to go there click here.