That's a terrible translation of 2 Corinthians 12:9. This is what it really says:

(2 Corinthians 12:9 NIV) But he said to me, "My grace is sufficient for you, for my power is made perfect in weakness." Therefore I will boast all the more gladly about my weaknesses, so that Christ's power may rest on me.

You're misinterpreting Proverbs 18:21. No, God doesn't give us power to change things just because we say words. This is what it says,

(Proverbs 18:21 NIV) The tongue has the power of life and death, and those who love it will eat its fruit.

Here's what one Bible commentary says about that verse:

Death and life are in the power of the tongue,.... Of witnesses, according to the testimony they bear; of judges, according to the sentence they pass; of teachers, according to the doctrine they preach; of all men, who, by their well or ill speaking, bring death or life to themselves and others. Some, by their tongues, by the too free use of them, or falsehood they utter, are the cause of death to themselves and others; and some, by their silence, or by their prudent speech and prevalent intercession, secure or obtain life for themselves and others; yea, judgment at the last day will proceed according to a man's words, "By thy words thou shalt be justified, and by thy words thou shalt be condemned", Matthew 12:37; the tongue is the instrument either of a great deal of good, or of a great deal of evil;