While it's true you'll find the phrase "life, liberty, and the pursuit of happiness" in the Declaration of Independence, you won't find it in the Bible. God did not put you here with the express purpose of seeking personal happiness. Now that is not to say He does not want you to be a happy, fulfilled person, because I think the most fulfilled people, the happiest people, are the children of God. But He did not put us here to merely pursue happiness.
So why did God put us here? First, we are here to know God and to glorify Him. As the apostle Paul said, "We who first trusted in Christ should be to the praise of His glory" (Ephesians 1:12). Glorifying God means that we try to honor Him in whatever we do. And when we succeed in certain endeavors, when we achieve, we must remember that God gave us the strength to do so. It all comes as a gift from God.
Secondly, we are here to bear fruit. Jesus said, "You did not choose Me, but I chose you and appointed you that you should go and bear fruit" (John 15:16). If we are true children of God, then spiritual fruit should be evident in our lives.
Now one thing about fruit is that it doesn't grow overnight. It takes time. And sometimes we grow impatient in our lives when we don't see the growth we would like to see. So don't be discouraged. Don't give up. Just stay rooted in Christ, and you will bear fruit, to the glory of His name.