2nd Chronicles chapter 13. Rely on God!

Abidjah was King over Judah for 3 years and there was war between him and Israel, King Jeroboam. They went into war with each other. Judah with 400,000 men and Israel with 800,000 men. Abijah stood before Israel the enemy and told them not to fight because God was not with them. He explained that God was with Judah. But was not with them cuz they served other gods. In the meantime Israel surrounded Judah and went into war. When Judah saw that Israel had surrounded them they cried out to God. It says they relied on God and God gave them victory over Israel. Judah was able to kill 500,000 of the 800,000 men.
I like what it says: they won because they relied on the Lord. It sounds so simple. But why is it so hard. There are days when I feel things are going good and trusting God is not an issue. but there are days when things go wrong and because I don't see the future outcome, I assume the worst. Then I don't act in faith, but instead I complain and worry and fret before God.
Lord help me rely on you when things are going good and when things are not going well at all. Amen.