Dear [Name], 

I wanted to thank you for all the support you gave me while working on the project. Without your help, I wouldn't have been able to finish it. Your expertise and dedication made it possible for us to reach the desired outcome. You consistently went the extra mile in order to get it done right and on time. You are an amazing colleague and a great asset to our team.

Thanks again for everything.

Sincerely, 
[Your Name]
Scrivi una lettera di ringraziamento a un collega per averti aiutato con il tuo progetto.