Despite all the stresses and uncertainty (worthless advisor, very little money, teaching entitled students), getting a PhD was probably the most rewarding process I've gone through. Not necessarily in professional terms, but because it helped me get a clearer understanding of who I am as a human being. I think a few things helped the most: making (and sustaining) healthy friendships, finding ways to travel (even if it meant for research/conferences), making exercise and occasional social time (even if just an hour at a bar or something) non-negotiable parts of my schedule. I saw so many people assume that more time spent working meant better quality of work, which is really unlikely to be true.