Two tips that will help prevent your soul from being sucked out and trampled:
1.) Try to identify whether the creative project you are working on is for work or pleasure. If you confuse the two, you're likely to become frustrated and burn out.
For instance, if you're trying to gain creative satisfaction from a commercial assignment that has no satisfaction to offer, you'll keep blaming yourself for not finding the joy, or you'll blame your client for not providing the joy, and eventually you'll throttle someone or jump off a bridge. You gotta face the fact that certain assignments are merely bread-winners. You're putting in the hours for a paycheck on those jobs, much like passing your day behind a cash register, and the sooner you stop expecting that special 'arty' feeling to magic things up, the more content you'll be.
By the same token, when creating for pleasure during your off hours, don't let your commercial instincts kick in. Thoughts like "publishers would like this if I add..." must be taken out back and summarily executed. You need those sessions to remain wholly your own, in order to recharge your batteries. Attempts to straddle the horses of bliss and commerce will result in soreness, and you'll wonder bitterly why you ever got into this field.
If you're nimble enough, you may even be able to make this distinction in the course of a single assignment, discerning aspects that are potentially pleasurable from those that aren't. You can then thrill over drawing the horse, and soldier through drawing the barn, and neither task will be compromised. The horse will bring you all the joy it should, the barn won't stink of unmeetable expectations, and both parts of the picture will look as good as possible.
2.) When working on someone else's project, look for your satisfaction in the process and not the result.
The end product in such cases will likely be jacked-up beyond all hope by the client. They'll choose an inker, letterer, or colorist who will urinate asparagus-scented pee all over your hard work, they'll add typos and garish icons, they'll print it upside-down and non-consecutively in red and green ink, they'll market it poorly, excise the good parts, and add Jar Jar Binks (for the kids!). They'll do everything to your work that Hitler did to Poland, savaging your efforts like dingos on a baby, and if you've motivated yourself 'til that point by thoughts of the end result, the smoke of your torment will rise forever.
Enjoy the process. You're building sand castles. Daily, the tide of your client's awful taste will wash away the beautiful object you just created, but that won't matter. You'll have had your fun and moved on.