God sent Jonah to preach destruction to Nineveh. The Ninevites took this seriously and repented. God “changed his mind” and didn’t destroy Nineveh. Does this mean that God can’t be trusted? That he just may change his mind midstream?

No, Jonah knew what would happen, and he was a bit miffed at God over it. He complained that if he preached, the people would repent and God would withhold his judgment, after all he is “a gracious God and merciful, slow to anger and abounding in steadfast love, and relenting from disaster.” Jonah didn’t want the judgment withheld. He didn’t like the Ninevites. Jonah had a problem of his own. (Jonah 4:1-4)

But God was true to his character. There is a higher principle. When people repent of sin, God forgives their sin and judgment is withheld. God was completely true to his character.

God is not man, that he should lie,
    or a son of man, that he should change his mind.
Has he said, and will he not do it?
    Or has he spoken, and will he not fulfill it? – Numbers 23:19

And also the Glory of Israel will not lie or have regret, for he is not a man, that he should have regret. – 1 Samuel 15:29

What about the case where God wanted to destroy Israel, but Moses prayed and “changed God’s mind?” (Exodus 32:9-14). Or the case of King Hezekiah who was sick and Isaiah prophesied that he would not recover but would die. Hezekiah prayed and wept bitterly to God and God sent Isaiah back to say he had been given another fifteen years?(Isaiah 38:1-6).

In each of these cases and a few more, what we see is God’s current disposition to a present situation. When the situation is changed, God responds to the change in accord with his character. People pray, people repent, God responds.

We can trust God’s promises, in particular, his promise to forgive our sin when we repent. He is fully trustworthy. God is the ultimate role model and deserves our ultimate worship and awe. It is because he is “a God merciful and gracious” that we can live with confidence and joy. As we come to know him better, our own character is transformed to be like him, changed from glory to glory by the Spirit of the Lord. Progressively being conformed to the image of Jesus.

Is it possible that you need to pray or repent thus changing your circumstances? God will respond in accord with his character, “a God merciful and gracious, abounding in steadfast love and faithfulness.”