Dear Amir,
I'm so sorry to say that I can't come on the trip after all. I know how long we've been planning it, and I hate letting you down.
The reason is that I've just found out that I have to take a resit exam on 14 July, which is exactly the day after we're due to leave. I thought I'd passed the chemistry paper, but my result came back last week, and I missed it by two marks. If I fail again, I'll lose my place on the course, so I really have to spend the next few weeks revising.
I'm so disappointed. I was really looking forward to the hiking and the evenings around the fire, and I feel like I'm missing out on the best part of the summer.
Please go without me and have an amazing time! When the exam is over, why don't we plan a shorter trip of our own, maybe a weekend by the sea in August?
Love,
Selim