The provided phrases are already well-formed grammatically. However, for clarity and style, here are some slight adjustments:

"Believe in what you think. Live by what you believe in."
"The destination is not as important as what you encounter along the way."
"Aside from being content with what you have, we must also find contentment in what we do not possess. Because the source of our suffering can be what we once had."
"Life is too short to spend on things you don't like. Choose what you enjoy, for time is limited."

Phrakru Samuphonthep Chantachoto