|Grace under pressure!|
It's the deadlines. Other people's emergencies that suddenly become yours. They've left things until the last minute and suddenly need you to work crazy hours so they fulfil their promises.
Being gracious is the best approach. Ranting and raving and complaining doesn't make for efficiency or sparkling writing and it wastes time as well as emotional energy. I know, because I've done that.
Going the extra mile with a smile on your face isn't the same as being a doormat. Not if it's your choice.
It gives you a good reputation and, yep...more commissions!
As long as you're confident you can say NO when it really doesn't work for you...