Summary of my situation:
I'm early in my 4th year as a PhD student (Cell Biology), and I think I want to quit. My program is 5-7 years, so I have at least 2 years left or I can drop out and claim a Master's and say that I have an extra year of experience since I qualified a year ago.
I have an exciting project that I'm interested in, but do not feel like my advisor lets me finish experiments before expecting me to start the next set. As a result, I have very little to show for a lot of work. The paper will probably be big, but it's far off.
I don't think I'm getting what I wanted out of grad school. I see ways that I can achieve those original goals in the future, but it will be a while and I'm not sure its worth the wait.
The stress and workload leave me racked with guilt, initiate panic attacks, and have sent me into depression (maybe? I'm planning on talking to someone at my school's student mental health center). I know it's work related because when I do manage to take a day off, I feel perfectly normal, like my old self, but as soon as I walk back into lab it starts again.
Can I talk to another professor (someone on my committee, someone I'm familiar with in my department, or someone familiar with my project but not on my committee) to get advice about my particular situation, the advantages/disadvantages of quitting, and how to talk to my PI about it? Or would that be seen as going behind my PI's back?
Talking to other people including mentors is always a good idea.
It sounds like you're having trouble handling the stress of a PhD and your desires to drop out are not related to any particular issues with your advisor. It's almost impossible to tell how your PI would take it. However, I doubt most reasonable people would consider you seeking advice from others as "going behind their back."