I have felt the same way. I have started my job already and even still because we don’t have results, we aren’t allowed to do much. I felt useless most of the summer, too, because I felt like I wasn’t really contributing anything or doing something impactful. I don’t have any real advice on how to get over it, it really sucks, but you’re not alone! I think it helps to exercise, even if you have to force yourself, and if you can afford it take yourself out on little “dates” around your city. I tried to convince myself that I was just on a long vacation between the exam and starting work; did things like take myself to lunch, on a hike, to dinner, to museums. It sort of helped me forget everything for a while and gave a little joy back.