Thomas á Kempis. The imitation of Christ. Book 2, chapter 5.

Of self-esteem

We must not rely too much upon ourselves, for grace and understanding are often lacking in us. We have but little inborn light, and this we quickly lose through negligence. Often we are not aware that we are so blind in heart. Meanwhile we do wrong, and then do worse in excusing it. At times we are moved by passion, and we think it zeal. We take others to task for small mistakes, and overlook greater ones in ourselves. We are quick enough to feel and brood over the things we suffer from others, but we think nothing of how much others suffer from us. If we would each weigh our own deeds fully and rightly, we would find little cause to pass severe judgment on others.

The interior person puts self-care before all other concerns, and those who attends to themselves carefully do not find it hard to hold their tongues about others. You will never be devout of heart unless you are thus silent about the affairs of others and pay particular attention to yourself. If you attend wholly to God and yourself, you will be little disturbed by what you see about you.