You will judge yourself as harshly as you judge other people. And to judge yourself without compassion, mercy and self-forgiveness is to condemn yourself to a life of misery, pain and self-flagellation.
Treat others with mercy, with kindness, and with empathy - you set a standard for which you will treat yourself when your own mistakes come, when you fall short of your own standards, when you detect inconsistencies in your own life between what you profess and what you do.
That’s what the bible’s Jesus was talking about in the first epistle:
“For with what judgment ye judge, ye shall be judged: and with what measure ye mete, it shall be measured to you again.”
He wasn’t setting a law (he wasn’t into that kind of thing). He was speaking about a posture. A way to live life with grace and wisdom. And thus, peace.
You’re human. You’ll make mistakes. The way you treat others who do is the way you’ll treat yourself when you do.
And without mercy, your life will be one of misery.