We allow ourselves to be tempted or to struggle, but in God, there is complete contentment, ALL THE TIME.
We often don't rest in God because of our pride, but it is better to rest (Heb 4:3).
God promised to fight our battles. You want God to fight your battles and so does He. Let Him work and you find your contentment ALL in Him.
Often frustration can come to us because we aren't letting God teach us that He wants to do things HIS way.