When is it time to quit?

Back story: I work for a wonderful non-profit in marketing part time. I do social networking and I write press releases and whatever else my boss doesn't feel like doing or doesn't have the time to do.

I love this organization, the problem lies in the way I am treated. Examples: I am only allowed to work 1.5 days a week in the office because that is all they can afford. I am not allowed to use this time to attend weekly staff meetings that everyone else attends, my boss has implied that it is a waste of time and money for me to sit around listening to people despite my arguments with her that I need to know what is going on for social networking purposes. I was asked to write a press release at home because it was urgent and it never got sent out to the press.

All of this is seriously starting to effect me to the point where I am miserable and I know I have to quit but I just keep making excuses: It is my only source of income right now. I am a graduate assistant part time in exchange for free school. My GA schedule changes every 3 months and I know it will be hard to find a job that will work around my schedule. I am kind of picky because I want a job in marketing to continue to gain experience in the field. I know that if I leave, no one will continue the social networking that I have worked so hard on. I care too much about an organization that doesn't care about me.

How do you quit your job with the only reason being that you are miserable and can't take it anymore? I can't tell them that, I need them as a reference.

Reddit please talk some sense into me. I've read other posts about how to quit your job, I just can't seem to get the guts up to do it. Am I crazy? Should I keep going until I'm done with school (16 months)

TL;DR I'm treated like crap at my work and I need to quit but I can't grow a back bone and do it.

EDIT: Thank you so much for your advice. I've decided to put in a resignation letter on Monday. We'll see how it goes.