Jeremiah 29:11

Act justly, love mercy & walk humbly.

You're misinterpreting the verse in Jeremiah. Look at the passage:

(Jeremiah 29:4-14 NIV) This is what the LORD Almighty, the God of Israel, says to all those I carried into exile from Jerusalem to Babylon: {5} "Build houses and settle down; plant gardens and eat what they produce. {6} Marry and have sons and daughters; find wives for your sons and give your daughters in marriage, so that they too may have sons and daughters. Increase in number there; do not decrease. {7} Also, seek the peace and prosperity of the city to which I have carried you into exile. Pray to the LORD for it, because if it prospers, you too will prosper." {8} Yes, this is what the LORD Almighty, the God of Israel, says: "Do not let the prophets and diviners among you deceive you. Do not listen to the dreams you encourage them to have. {9} They are prophesying lies to you in my name. I have not sent them," declares the LORD. {10} This is what the LORD says: "When seventy years are completed for Babylon, I will come to you and fulfill my gracious promise to bring you back to this place. {11} For I know the plans I have for you," declares the LORD, "plans to prosper you and not to harm you, plans to give you hope and a future. {12} Then you will call upon me and come and pray to me, and I will listen to you. {13} You will seek me and find me when you seek me with all your heart. {14} I will be found by you," declares the LORD, "and will bring you back from captivity. I will gather you from all the nations and places where I have banished you," declares the LORD, "and will bring you back to the place from which I carried you into exile."

God is promising to bless Israel as a nation in the future despite the fact that the Israelites are being taken to Babylon as slaves and the vast majority of the population will die there. You can't apply that to individual people in the church age. God never promises prosperity to Christians.

God’s wonderful plan for our life is that we repent, believe in him, and fight sin through his power. God’s wonderful plan for our life is sober-mindedness, sexual purity, and that we “walk not as unwise, but wise, making the most of our time because the day is evil” (Ephesians 5:15-16). God’s plan is that he may be most glorified in us through us being satisfied in him.

Jesus promised that if you come to him with a prayer, he will lift you up.

As Christians we know that true happiness — the truly wonderful life — comes from true surrender.