1．I want to expand my horizons.
2．I've made a tough decision, sir. Here is my resignation.
3．I quit because I don't want to be stuck in a rut. I want to move on.
4．First of all, I'd like to say that I've really enjoyed working with you. However, I think it's about time for me to leave.
5．I've been trying, but I don't think I'm up to this job.
6．I've been here for too long. I want to change my environment.
7．I'm sorry to bring up my resignation at this moment, but I've decided to study abroad.
8．To be honest, I've got a better offer.
9．I'm running out of steam. I need to take a break.
10．I'm quitting because I want to try something different.