誰にお別れを告げているのか分からないので難しいですが、FormalとInformal、二つ書いてみました。
Formal(会社など)
I'm writing this email to say goodbye to you all.
Although it was only for a shot amount of time, I am really grateful for your friendship and kindness I received.
It seems like only yesterday we were travelling and having meals out together.
I had a wonderful time getting to know you, and it fills me with sadness to know I will soon be departing.
I am sure we will meet each other sometime soon, and when we do, please feel free to say hi.
Informal(友達)
I just want to say goodbye to everyone.
Thank you for being so kind to me during this short period.
It seems like only yesterday we were travelling and having meals out together.
It's really sad to have to go after getting to know you all.
I am sure we'll bump into one another sometime in the near future, and when we do, be sure to say hi!
お礼
早速ご回答頂き有難うございました、時間がなくて助かりました。 また説明不足ですみませんでした、会社関係です、有難うございました。