The only time is it legitimately acceptable to not forgive is — never. Yes, it was a trick. Do you forgive me? Peter wanted to know how many times he had to put up with forgiving a brother (Mat 18:21), but he didn’t understand why he had to forgive.
We don’t forgive others because we’re being nice or letting them off the hook. We forgive others to clean house in our hearts. Unforgiveness is rottenness to our soul. If we leave it there, it festers and grows and damages us from the inside.
Remember that forgiveness doesn’t mean we condone bad behavior, it just means that we’re not going to hold onto anything that hurts us. We forgive them and trust God to deal with them.
Forgiving ourselves is just as vital as forgiving other people. Confess your sin to God. He will forgive you, so you should too. If you refuse to forgive yourself after God already has, then you have some issues that He needs to help you with. Lay it all before Him and ask Him to clean you up. He can fill you with His peace and give you joy. That’s His desire for you. Just forgive and let blessing flow.