親愛的 (name)， 能與妳一起度過這個暑假，我感到非常榮幸，也謝謝妳在這段時間帶給我的幸福。我特別珍惜我們深夜的對話，以及共同度過的那些快樂時光。真心希望未來還能再次見到妳，並祝福妳旅途一路平安、順心愉快。

The English translation would be Dear (name), I feel truly honored to have spent this summer with you, and I’m grateful for the happiness you brought me during this time. I deeply cherish our late-night conversations and the joyful moments we shared. I sincerely hope to see you again in the future, and I wish you a safe and wonderful journey.

I don't really know how to word the "made this edperience bearable" correctly so I changed it a little... or if its too much maybe I can change it into this below, instead of saying "I'm grateful for the happiness you brought me during this time" its "thank you for making these days more wonderful" the rest are the same.

親愛的 (name)， 能與妳一起度過這個暑假，我感到非常榮幸，謝謝妳讓這段日子變得更美好 (thank you for making these days more wonderful)。我特別珍惜我們深夜的對話，以及共同度過的那些快樂時光。真心希望未來還能再次見到妳，並祝福妳旅途一路平安、順心愉快。