Knowing your limits and setting boundaries
You know yourself better than anyone else. When you are not doing the art business full time (or don't have a lot of support), you are doing it around other things - family or work or illness. You have to be realistic about how much time you actually have for your business... and then think about how much time you have and whether you can actually commit to every spare hour you have.
I get home from work quite often after 7pm at night after working a standard day. I have one day off a week that many weeks is half taken by doctors appointments. After getting dinner and getting my crap together for the next day (in theory there's cleaning dishes, maybe doing a load of washing) I am left with not a huge amount of time. I can't stay up late. My meds make me tired, plus I need sleep or my condition deteriorates. I sometimes will answer emails, do some social media, write a blog post, pack an order... but most of the time I've learnt I need to have wind down time for myself. I might game for half an hour or watch some TV or read a book. Years ago I used to come home, and paint or study until midnight. Nowadays I can't.
Knowing my limitations, I've come to the realisation that I don't work quick enough for freelance gigs like game cards or company illustrations. It's why I do private commissions because I have more control. I've only done one illustration for a game when I was overwhelmed and probably didn't have the energy. It took longer than I wanted (though there was no strict deadline), but I always felt bad about it.
What are your priorities
Priorities can shift at any time of the year, at any time in your career. Your business has to be fed and nurtured, some times it will take more energy than at others. You have to make a commitment to it. But, that doesn't mean your priorities are going to be consistent. When I first started, prints and products were my priorities. I had to change my priorities when the margins became really narrow and I worked out I effectively had 3 hours on a Saturday morning in which I could get to the post office. You would think this is plenty of time, but what happens when a family event comes up, you get sick, you have to go to a shop that's open in the same time but on the other side of town.
Priorities also are about what makes you happy or gives you satisfaction as an artist. You're compacting your art business time down into maybe a few hours each night and possibly part of the weekend. You will begin to resent your business if you feel like you are coming home to more work. Or you'll burn out.
Think about what it is about the art business you want to focus on and don't apologise for it. As long as you are professional, carefully consider what does and doesn't align with your priorities.
One of the things I did badly when I started was reacting to things, rather than being prepared in advanced. I'm moving my print sales pretty much to fulfilment as the time cost for packaging items and going to the post office, even for one item is worth less to me than time spent painting. But while doing lots of print sales I found that sitting down and preparing a stockpile of prints, buying backing boards in the right size rather than hand cutting to size, and using prepared templates instead of a custom one all save time and energy. I used to have stickers for shipping labels but these days I've changed directions so don't do huge amounts of physical sales.
For commissions, I have contract templates, I should have canned email responses but I don't. I do however try to make my information about commissions clear and concise. Consider having a FAQ page... I think mine hit the dust when I rebuilt my website, but I did make a post about image usage because that is what I used to get the most emails about when I was first starting out.
Also set up client expectations up front early. I have worked out I have to chunk work into thumbnails, sketches, and then the final painting. I give an estimate for the work explaining roughly when I'll be checking in, and try to stick to it. Unfortunately things don't always go to plan as you can't prepare for being sick or your computer dying, or thunderstorms preventing you from working digitally the only day you have off!
Learn to say NO
I cannot stress this enough. If a commission feels off, don't take it (working with a day job means I can be a bit choosy). I still take on work that occasionally the interest wears off on part way through, but I've learnt that some work or some opportunities can be energy vampires. When your time is limited, you do not want to be stuck doing something that takes up time you don't really have and makes you resent your creative business. Most of the time something else will come along that better aligns with your priorities anyway.
If at first you don't succeed...
There are dozens of techniques for improving productivity. Pomodoro, time-boxing, those tools that eliminate distractions, Rescue Time (for watching how many hours you spend avoiding work...) Try each one, and if it doesn't work, move on. Just be prepared that some things will work, some things won't. You are trying to work smarter, not harder.
Build in down time
You are not a machine. You need to have fun. It is healthy to paint your own personal work, go to the movies, have family days, go on holidays, exercise (bleh), do things that you enjoy. I probably paint less these days because I game more than I used to when I first started my business, but I need that time where I'm not thinking about work. It's my thing that I look forward to. You have to have something you enjoy doing outside of work and family.
And don't feel guilty about it (unless it's interfering with deadlines and you're procrastinating)