Worth a read

I'm sure everyone has read it already, but if you haven't read Derek's comments about depression and anxiety in graduate school (complete with his own story of late night failed experimental runs), you should. The last two paragraphs are worth quoting in full: 

...I think it’s important for graduate students to realize that everyone has these doubts and bad stretches. Everyone has these moments when they wonder what they’ve done to their lives, but having these thoughts is not a sign that the exact failure you’re fearing has arrived. That doesn’t mean that thinking about your purpose in grad school is a bad thing, but it probably is a bad thing to try to do it at periods of peak emotional stress. If you feel that it really is getting too much, definitely talk to someone. Universities have people around for just that purpose – more so than in my day, fortunately – and if you find yourself wondering if you should reach out like that, then odds are that you should. Do it. I wish some of the people I worked with had, or had been able to. 

Any meaningful graduate degree is going to be a test of your abilities and your resilience. Recognize this, and avoid the two extremes. On one end are the macho types whose response is “Eat stress for breakfast! That’s what I did in my day! If you don’t have the fire in your belly you don’t belong here!”. And on the other end are the voices, some perhaps external and some internal, telling you that you’re a failure already, an imposter, and that you’re never going to measure up anyway. These are two different sets of lies, and everyone has to steer their course between them.

This is the sort of thing that we should be reminding 3rd years and 4th years in graduate school on a regular basis. Derek's been there, I've been there, probably your PI has been there too. It's okay to ask for help.