Dear [Recipient's Name],

I am writing this letter to express my heartfelt gratitude for your kind assistance during my recent project. Your support and guidance were invaluable, and I truly appreciate the time you took to help me achieve success. Without your help, I would not have been able to complete the task as efficiently.

Thank you once again for your generosity and encouragement. You have made a significant difference in my life, and I will always remember your kindness. Please accept my sincere thanks.

Sincerely,

[Your Name]

This simple message conveys deep appreciation while maintaining brevity.