Sounds like the client sucks if you can't slow down or stop or control anything, dump them or start saying No. If you are going to lose out to the competition by saying No, then it sounds like you aren't providing any real value, you are just cheap labour that is replaceable and are getting paid poorly and incorrectly for it.
The point of being self employed is so you can do whatever you want and own your life. If you get good clients then you can get paid more, they can't afford to lose you or easily switch because of the value you provide and are willing to work with someone who has reasonable human needs. You can take projects without hard fixed deadlines or with vacation time booked in and get paid on the project, not by the hour or even day. If you provide real value, they'll want you back for the next thing.