Whether in business or in your personal life, relationships are complicated (I should know, considering I met my wife AT work). So when it comes to my career as an independent contractor, those business relationships are my bread and butter - literally.
I enjoy generally positive, enriching, fulfilling relationships with most of my clients - I write stuff for them, that stuff helps them sell other stuff, they get some money, I get some money, they trust me with the next bunch of stuff that needs to be written, yadda yadda yadda.
Just as in life, some relationships don't always go the way you want, or the way you expect. And even the seemingly best clients can end up going sideways on you with one bad project. Don't shake your head. It's not a matter of IF it will happen, but WHEN. How you handle it will determine whether or not you end up putting "former" in front of "client."
So, what do you do when your client is suddenly uber-critical, hyper-demanding, and generally making you wish you never took this on? These three tactics have helped me immensely:
Clients are people too. With bosses. Who are stressing them out as much as they're stressing you out. Whether it's a long-standing client who's never acted this way or a first project with a new client, try to figure out what's underneath it - a deadline they have to meet? Pressure for this project to be a success? If you understand where they're coming from you may be able to work together to find a solution - and help nurture the relationship if they recognize that you're trying to see things from their point of view. This goes under "The client may or may not be right, they're just too wrapped up in other things to be of any help."
Just because your clients may not be treating you with what you might consider professional courtesy doesn't mean that you have to follow suit. Don't argue. Don't get pulled into a confrontation. All of those routes will impact your reputation, and could affect your bank account if the troublesome client withholds payment. Hold your tongue, maintain your commitment to finishing the project, and when it's complete, politely decline any future projects from that particular client. (So yes, even if you vehemently disagree, this still falls under "the client is right.")
If the situation is becoming unbearable and points 1 and 2 aren't getting you anywhere, it may be time to move on. But as an independent contractor, this should be an absolute last resort since you can't afford to burn any bridges. Your exit must be both strategic and professional. Give fair notice (two weeks is still appropriate), let the client know why you'll be unable to finish the project, explain that you'll be billing for time and materials up until the termination date (as long as it's in your independent contractor agreement), and offer to help transition the project to whomever the client hires next. Understand then that you cannot control or predict their reaction. All you can do is conduct yourself professionally, and hopefully move on without further incident. This is the dreaded least appealing "the client is wrong, go nuclear" scenario - the one you want to avoid at all costs.