You're tired and worn thin. It happens to everyone, regardless of stutter. The stutter is just another way for it to happen. It also sounds like your have a difficult advisor, which is another source of stress. A PhD is easily going to be another 2-4 years of work. That's a long time to be run ragged. Make a point to take care of yourself and set limits around your work. Sometimes you will have to put in 16 or 20 hour days, but don't let it be "normal." Take the occasional day off (or at least "light" day) to spend time with friends. About friends: You care much more about your stutter than anyone else does. Don't fall into the trap of believing that other people view it as a negative. In fact, the people that love you probably find it endearing. I understand the feeling of "this isn't worth it," or "can I do this." Do the thing that you dream to do, no matter how hard it is. It's clear from your writing that you won't be satisfied with anything less.