Everything we do is wrapped up together in our lives.
It's impossible to completely separate out the different parts.
Finding positive motivation to move forward with a project is essential--making the project something we believe in, something that seems important--without the sense that what you are doing is important, it's very hard to find motivation when the going gets tough.
Therefore, it is important to recognize the negative motivations that are influencing your actions.
If you think of your Ph.D. as nothing more than bullshit Piled Higher and Deeper, then you're creating a psychological space in which the work you've chosen for yourself is equated with shoveling manure. If that works for you, hey, right on. But most people aren't that excited about shoveling shit.
But you can do something about that: you can try to refocus on the things that you felt were important, the things that got you into the world of academia in the first place.
But even if you believe in your project, you can still kill your own motivation by focusing on your weaknesses.
What? You haven't read all the relevant articles on your subject?
You don't speak a certain language well enough?
You don't think that you can do work as good as the people whose work you want to emulate?
That kind of thinking will kill a project. It takes time and emotional energy and leaves one in a state of doubt.
Motivation grows when you focus on your strengths and on what you believe is important.
So try to recapture the enthusiasm that came at the start of the project: what were the big goals that you hoped for? What were your abilities that made you enter into the project? By looking for your strengths, by focusing our attention on them, you have the dual benefits of developing your project in the path most likely to lead to a fruitful result, and bolstering your emotional state by focusing on things that you can do.
Does a sprinter try to prove his/her prowess by running a marathon?
Sure, versatility is great, but if you want the gold medal, doesn't it make more sense to run a race that you have excelled in in the past?
Use your strengths and work from them.
Don't tell yourself you don't know as much as person X; don't tell yourself that you haven't read the entire material. Instead, focus on what you do know, on what you have read, and look for the ways that you can bring positive motivation to your project.