Positive thinking is of no use. Being optimistic is just another way of saying that you’re lying to yourself (only seeing the good side of things). The Gospel is not based on optimism but on facts — not seeing the rosey side of things, but seeing the full scope of things and how good all of it is.
But despite the fact that the Gospel is not the same as positive thinking, the Gospel certainly leads to positive thinking. You cannot say that you’ve seen what Jesus has actually done and not feel entirely positive about your life. Don’t think positive, think Jesus. That will lead to feeling positive all by itself.