i try to keep the bigger picture, too. just sometimes, the bigger picture is a bit depressing (why am I doing the PhD at my age? because i want to try for a career in academia. so i can't lose any more time. so i need to be very fast and efficient now and do everything at once!)
but i am staying sane, i believe, most of the time anyway. in some sense the PhD helps me to keep my sanity. in my last 8-5 job i had quite some trouble with sleeping, which didn't help with my mental health. now, when i can't sleep, i stay at home longer in the morning. i get the same amount of work done but at different times and am not over-tired all the time.
i also had a job at some time, with a lot of responsability, for a company, for people. that was very stressful. now, doing the PhD, i am responsible to myself and my research, that is a lot more manageable and indeed helps to keep me sane!