"For he that will love life, and see good days, let him refrain his tongue from evil, and his lips that they speak no guile:" 1 Peter 3:10
"Death and life are in the power of the tongue: and they that love it shall eat the fruit thereof." Proverbs 18:21
The apostle Peter plainly tells us that enjoying life and seeing good days, and having a positive mind and mouth, are linked together.
Our mouth gives expression to what we think, feel, and want. Our minds tells us what we think, not necessarily what God thinks. Our will tells us what we want, not what God wants. Our emotions tell us what we feel, not what God feels. As our soul is purified, it is trained to carry God's thoughts, desires, and feelings; then we become a mouthpiece for the Lord!
Your words, as reflections of your thoughts, have the power to bring blessing or destruction not only to your life but also to the lives of many others. In 1 Corinthians 2:16, the Word of God teaches us that we have the mind of Christ and that we hold the thoughts, feelings, and purposes of His heart. We hold them in us, but the flesh often blocks them from coming forth. Therein lies the battle -- the continual struggle between our flesh and the spirit.
No matter how negative you are or how long you have been that way, I know you can change because I did. It took time and "heaping helpings" of the Holy Spirit, but it was worth it.