I will share some of my journey, to give perspective on a critical spirit. I underwent many years of rejection, feeling both rejected by people and God. This built up hurt in me, and out of that hurt, I would often speak negatively. Not of others, but just negatively of situations. This is not to say I never thought a negative thought of other people, I actually harbored a lot of jealousy in my heart toward others, it may have caused me to lash out, if not in words at least in spirit. It was only as God healed me personally of these wounds that I was able to get a positive outlook. Sometimes our situations need to change to have a change of heart.
However, there are also times we need to take captive the negative thinking that can invade our space. But that is more a spiritual battle; fighting off unwanted thoughts. For me, this involved speaking back the word of God to the thought. If I felt like hating someone for the hurt they had caused I had to speak back continually, "No God says to forgive, and love them". In the end, after a long time of doing this, the thought would stop.