To fulfill the mitzvah of bitachon (trusting Hashem) is to know that whatever happens is being done by Hashem for a kindly purpose.

You may not know what that purpose is; it make take years to discover; it might be necessary to go to the next world before you discover what happened. But you someday will realize the truth of this principle: that Hashem does everything only for benevolent purposes. ~ R’ Avigdor Miller zt”l