For the Lord to help you form more positive thought patterns, you must be willing to change.
Psalm 51:2,7,10
Romans 12:2
Ephesians 4:22-24

Stop yourself at the first negative thought.
2 Corinthians 10:3-5
Ephesians 4:27
James 4:7

Fill your heart and mind with God's Word.
Joshua 1:8
Psalm 1:2
1 Timothy 4:13-16

Take a positive approach to tests and trials by seeing the Lord's hand in them.
Psalm 119:71
Romans 8:28
1 Thessalonians 5:18
James 1:2-4
1 Peter 4:12-13

Cultivate positive attitudes towards others.
Romans 12:10
Ephesians 4:32
Colossians 3:12-14
1 Peter 4:8

Promises and prayers to help overcome negative thinking:
Isaiah 55:7
Psalm 19:14
Psalm 139:23-24
Proverbs 16:3