אל תאמין בעצמך עד יום מותך don’t trust yoursef until the day of your death. The Chasam Sofer explains the mishna in the beginning of Pirkei Avos, when it comes to others, הוו מתונים בדין, wait with their judgemet by looking at the good in them whereas when it comes to youself, don’t trust yourself and crreate many students and protective barriers.
The mishna there (2,5) is explained by the Midrash Shmuel that iits answering the four excuses why people don’t want to join a tzibur.
1. I am better than them — don’t trust yourself.
2. They are no good – don’t judge your friend until you put yourself in his place.
3. They don’t want to listen to me – don’t say something that is not acceptable.
4. I join later – don’t say, I wll learn later when I have the time because maybe you will never have time.