If your mind is filled with defeat thoughts, fear thoughts, resentment thoughts, you are bound to be in a state of mental unrest, even turmoil, and of course there can be no inner peace. This passage advises you to practice thinking about God, to keep your mind "stayed" or fixed not upon your troubles but upon God. Keep your mind on God for as many minutes during the day as possible. This may be difficult at first. Practice will make it easier.
Thou wilt keep him in perfect peace, whose mind is stayed on Thee.