You can say no.
I went to school for graphic design, I was a bit precocious so I started it when I was 17; I had just moved out of my parents' house and was living on my own, the city was grand and the university a dream (at least in my eyes it was), I was ecstatic to learn everything about design... Unfortunately I didn't; as much of a cliché as this sounds, it's factual that there is a LOT about design you don't learn in the classrooms.
There are plenty of things that a professional graphic designer should know and/or have a basic know-how of, such as pricing, consulting fees, graphic royalties, pricing in tiers, and so on... But one of the most important lessons that no one teaches in universities is saying no.
Graphic design students go into school with a notion of what a designer is, a preconceived idea of what, who and how they develop in the real world once they receive their diploma, and from personal experience I can say it's wrong... And right. Society sees graphic designers as 'cartoonists' or 'doodlers', which is why they end up regarded as commodities instead of necessities; so, designers are left to beg, since their career is thought of something that just anyone with a computer and a specific software can do. I won't deny it, pretty much anyone can do it if they get a grip of the software, but this doesn't mean that the final result will be any good.
So, because our work is regarded as easy and not necessary, people assume it's cheap, or that it has to be cheap or negotiable; IT IS NOT; and then, because everyone else sees you as expendable, you begin to believe it, so you start to think that your work is pretty much the same as everyone else's, you begin your professional career agreeing and accepting any type of work, for any kind of money, because, in your mind, jobs are scarce and therefore, you must take all that you can get.
You should take all the work that you feel like doing, and that's being paid accordingly; you have to be mindful with your time and realize that you don't have to do it all, you need to get out of that mentality of "they're doing me a favor by doing business with me"; you're doing yourself a disservice by thinking and acting that way; your time and your work is valuable. Say no, no to work that you can't manage, say no to work that is not being paid fairly, say no to work that you just don't feel like doing because you find it boring, you can say no.