Dear John,
I am extremely sorry to tell you that I am unable to go to your party this Sunday. It is very kind of you to invite me to your house and I really look forward to going to your party and sharing your happy experience abroad.
Much to my regret, I will be absent your party, owing to the fact that the College Entrance Exam, for which I am now really busy preparing, is just around the corner. What's worse, I have caught a bad cold lately and I am really not myself. Anyway, we will have many more chances to get together. Shall we make it after the Exam? I am truly sorry. I hope you will forgive me and accept my sincere apology.
Enjoy yourself and wish your party a great success.
Sincerely yours,