Join the club I guess. I know how you feel and I'm going through exactly the same thing. Luckily I'm done my course work, but my project seems to be going nowhere and the results I get are mediocre. For the last 2 months the simple experiments that I did everyday just stopped working, literally just stopped working at all and I have no idea what the hell is going on as I've kept everything the same. I feel terribly incompetent, as though everyone else is grinding out results and publishing, while I'm just struggling to get the simplest thing to work.
On top of that I have feelings that my PI doubts me and thinks I'm incompetent. He also said I have communication problems and almost didn't accept me as a PhD student because of it. To make things worse, I moved away from home, from all my friends and my girlfriend. I'm in a new city and haven't made any new friends, and I'm in a long distance relationship. I'm going broke and live in an empty apartment with nothing but a mattress, a small laptop desk, and a chair. I can't cook, nor have time to, so many of my meals consist of milk and oatmeal. I would be lying if I said I'm not completely miserable.
But I am hopeful things will get better. I think this is just something all grad students have to go through. Grinding through experiments that don't work, resolving conflicts with your PI, and finally getting some kind of balance in life.
Cheers hopefully things get better for us all