Because forgiving others is the hardest thing for humans to do.
Discussion
I agree it can be very difficult. But if you are scared it is because you don't think you can do it. You need to pray. God won't allow you to go through any temptation you cannot handle 1 Corinthians 10:13
"No temptation has overtaken you except what is common to mankind. And God is faithful; he will not let you be tempted beyond what you can bear. But when you are tempted, he will also provide a way out so that you can endure it."