Thankfulness and contentment go together. But while they go well together, they are not the same thing. Part of being thankful is being glad for what you have. Part of being content is being glad with what you have.
The Bible tells us to be thankful (Psalm 100:4; Colossians 3:15). The Bible also tells us to be content (1 Timothy 6:8; Hebrews 13:5).
The Bible also tells us how to be both of these things. The secret to being thankful is realizing what you have. You have life and salvation, and access to the riches, strength, grace, and presence of God (1 Chronicles 29:12-13; Psalm 18:2; Psalm 28:7; 2 Corinthians 12:9; Philippians 4:11-13 & 19; Hebrews 13:5). The secret to contentment is found in the strength of Jesus Christ (Philippians 4:11-13).
And no matter what, God will always be sure you have what you need (Psalm 37:25; Matthew 6:31-34; Luke 12:30-31; Philippians 4:19).