There is a popular expression in society: "Don't leave anything to chance." This statement contains an implicit expression of arrogance and lack of awareness of the Almighty.
If your peace of mind is dependent on being certain that you have complete control over a situation and all the possibilities that might arise, you are bound to be disappointed. It is arrogant (as well as an impossibility) to think you have the omniscience to foresee every inevitability. To demand the impossible is to guarantee frustration.
Always realize the unexpected can occur. Plan as much as is appropriate, but realize that regardless of how much you plan there will be always be difficulties you had not previously imagined. By "expecting the unexpected" and being willing to accept it, you will have much greater peace of mind than if you unrealistically expect complete control.
(see Michtav M'Eliyahu, vol.1, p.188; Gateway to Happiness, p.83)