Let's begin by acknowledging the obvious: you simply cannot be all things to all people. You lack the time and capacity. Choice is essential, and so is "no." What you do and don't do depends on both who you are, meaning your values and the things you want to prioritize at this time in your life (money? family? free time? creativity? freedom?). You have to make choices that align with your values -- otherwise, you risk being false, fake, inauthentic, someone you'll end up loathing. Be you, always.
Time is also important. LIke you, I have limited bandwith. Time is perhaps our most precious resource. I can't tell you what to do -- only you can do that -- but I would suggest that you do work you like doing with the limited time you have available. When you do things with passion and enagement, you work better and smarter. You become more valuable. If you have enough of the type of work you like doing, i.e., if you're working at full capacity, feel free to say no to work that doesn't make you happy, align with your values, or allow you to learn something new or connect with people you like.
People are important too. Whenever possible, work with people (clients, co-workers, partners) who value the things you value. I always know when I'm working with someone who views me as a commodity, a human resource to be leveraged. I prefer to work with people who see me for what I am -- a human being (like they are and like you are). I'm not afraid to bring my humanity into my work. In fact, my humanity is what makes my work unique. People who don't work in a human way are fine with me. I work with many of them, but I most enjoy being human and working with others who are also human.
So when do I say no to writing assignments? First of all, I say no when I'm working at full capacity on tasks I enjoy. Second, when the people proposing the assignment haven't done the work of getting to know me or allowing me to get to know them. I have a hard time saying no to humans, but it's easy saying no to "humans" hiding behind job titles, people playing roles (not themselves), robots (human or mechanical), corporations (I don't care what anyone says, corporations are not people), institutions, people asking "on behalf of" organizations, etc.
I also say no when the relationship between the amount of work expected and the payment isn't favorable. I love to work hard, but I also love working with clients who respect and reward my passion and hard work.
For me to be creative, which every clients expect, I need to be happy and human. Money helps, but it's not the only thing. There's pleasure in the complexities of the creative process, in making something new and useful. There's the enjoyment I get from learning new things, from talking to smart people about important ideas. Each assignment offers unique opportunities, and I consider everything before I say yes or no. I ask myself: What's my relationship with the person making the request? What can I learn? How much creativity will be needed? How much work is involved? And am I ready to invest the time needed? How much does the assignment pay? Will it help me get more of the kind of work I want to do?
So saying no is complicated but necessary. I recently said no to a prestigious client because I have enough work on my plate right now and, as the Holidays approach, I'm not willing to overstretch and stress myself out (not for money, not for anything). Saying no is tough, but if you want to have any authenticity and integrity, you need to say no.
I've found that the things I don't do define me as much as the things I do. Saying no simply validates your limitations (we all have them), your values, and your creative preferences. It can feel empowering. Saying no also says "I have choice, agency, and I'm asserting my values."
The worst thing any creative person, or any person, can do is to try to be all things to all people. Be true to who you are. Saying no is also a way of saying yes to what matters most to you, preserving your time, your energy, your values, your health, and everything else that makes you special.