I want to expand my horizons.
I've made a tough decision, sir. Here is my resignation.
I quit because I don't want to be stuck in a rut. I want to move on.
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.
I've been trying, but I don't think I'm up to this job.
I've been here for too long. I want to change my environment.
I'm sorry to bring up my resignation at this moment, but I've decided to study abroad.
To be honest, I've got a better offer.
I'm running out of steam. I need to take a break.
I'm quitting because I want to try something different.