Having kept a journal (of sorts) for the last four years, it is interesting how I always look back on the 'dark' days most fondly, probably because having survived it I do feel mentally more resilient and stronger.
What startup are you working on at the moment? I find it very useful to have a 'co-coach' (not life coach), someone who shares similar experiences or is in a similar situation that can keep you accountable and motivated to get through the tough times.
It's tough going through a rough patch, but it takes even more strength to be able to share your vulnerabilities. At least every time you are pushed to the limits mentally and physically you grow stronger :)