I'm sorry you could not meet with the appointed time, because I will be busy preparing for an important exam. This is entirely my fault, missed a party so rare, I'm sorry.
I'm sorry I didn't meet you at the agreed time, because I'll be busy preparing for a very important exam. It's all my fault, I'm sorry to miss such a rare party.
I'm sorry I didn't meet you at the appointed time, because I will be busy preparing for a very important exam. It's all my fault. I'm sorry to have missed such a rare party.<br>