I believe in myself

You already know how to do this for other people. When a friend says they're going to try something, you don't run a risk assessment aloud. You say they'll be fine, and you mean it, on evidence that would never survive being applied to yourself.

The asymmetry is close to universal. The standard of proof we demand from ourselves would sink any friendship inside a month.

There is no trick for closing it, only a habit. Hear the internal verdict, then ask what you would say if that same sentence had come from someone you liked. Then say that instead.