Everyone who goes through PhD has to overcome fraustration and depression. It applies to every genious or average people like you and me. My professor says that the PhD is all about learing to be able to  overcome these obstacles in our life by being focused, dedicated and acquiring a critical thinking. Persoanlly, I don't care whether I will be a great scientist  in my field or not. I think u should know what you want to do in ur future. Will this degree helps somehow or not? If it doesn't at all, then you are wasting your time writing a thread. You should quit it.

Personally, sometime I feel like being a Monk but that is not the solution to what I want to be in my life. I can be a good teacher and I can sell myself with what I achieve. It's all about what is your exectation out of PhD. If you have high expectation about GC, house in US, BMW and bla bla bla. Those are all pushing you back. My roommate garduated and got a job that pays 90-100K per year( we had the same major). If I keep thinking about that and what I have at the moment, I will never be able to focus on what I want in my future. So, I keep pushing myself to the point I can take it. It's been 2 and half years already :) and as you are I don't know when I will graduate.