I want to expand my horizons. 
我想拓展我的视野。

I've made a tough decision, sir. Here is my resignation. 
长官，我做了一个很困难的决定。这是我的辞呈。

I quit because I don't want to be stuck in a rut. I want to move on. 
我不想陷入窠臼所以才辞职。我希望能往前迈进。

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.
首先，我要说的是，我真的很高兴能与你共事。但是，我觉得该是我离开的时候了。

I've been trying, but I don't think I'm up to this job.
我一直很努力，但我觉得无法胜任这个工作。

I've been here for too long. I want to change my environment.
我在这里待太久了。我想转换一下环境。

I'm sorry to bring up my resignation at this moment, but I've decided to study abroad.
我很抱歉在这个时候提出辞呈，但我已经决定要出国念书了。

To be honest, I've got a better offer.
老实说，我有一个更好的工作机会。

I'm running out of my steam. I need to take a big break.
我精疲力竭了。我需要好好休息。

I'm quitting because I want to try something different. 
我辞职是因为我想尝试些不一样的东西。