Happiness and God's Purpose

Such a trivial assumption, when we ask, "Doesn't God want me to be happy?" Our personal happiness isn't where God starts.   Though, He is a good God, as when we yield our lives and purposes to Him, a fruit that results is joy and peace, among others.  God calls individuals for His distinct purpose, which may or may not immediately manifest as  stereotypical "happiness" or comfort in you, in your life as you planned it. But its results are eternal.
Certainly, when He spoke of Abraham, he said, "For I have known him, in order that he may command his children and his household after him, that they may keep the way of the Lord, to do righteousness and justice, that the Lord may bring to Abraham what He has spoken to him," which was his son, Isaac, in the years of Abraham and Sarah's old age, to produce the Hebrew people, through which God would bring about His intentions for the world.  God had his Own purpose, and He chose Abraham  through whom to achieve it. (Gen.18:19)
God did not blow sunshine on Paul when he got his attention by blinding him on the road to Damascus.  "But rise and stand on your feet; for I have appeared to you for this purpose, to make you a minister and a witness both of the things which you have seen and of the things I will yet reveal to you." (Acts 26:16)  God chose Paul as a witness - not so he could make him feel good and find stereotypical happiness with a life of no challenge.  Paul faced many challenges - yet as expressed before, so also came joy, as a result of his obedience to, and participation in, God's purpose.
Each of us may choose to live out our life reaching for what we, in our limited vision, perceive as happiness; or giving the wheel to One who sees all, and can make us part of a story that is far bigger than ourselves.

Poison

Fitness to a task (or a relationship) - it's there or it's not.  It's not always that another person is a horrible monster. Sometimes it's that you yourself are not fit to abide them.  We're all on this earth for a purpose. We must find our purpose, what we are fit to accomplish, and apply ourselves to the task.
When we take on a task or relationship that exceeds our strengths, the task/relationship can poison our effectiveness, thwart our progress, and leave us damaged, unfit for our God-given task, which then does not get done, or done well.
It is not a failure to acknowledge our limitations, but a triumph, and a necessity.  We may wish we were fit to a particular task, but we abandon our real mission if we allow chasing the diversion to weary us into ineffectiveness.
Don't give in to the tendency to see it as failure to reject the task or relationship in favor of your primary purpose.  See it as having been led to the best path, while rejecting what was never meant for you to begin with, or which now exceeds your ability to  maintain.