... but that doesn't mean you can't try.
First, be fully prepared to lose her permanently from your rotation. Never forget that she's with you for cash - no matter what else you do together or what she says she likes about you. So you're only levers here to reduce your total spend on her are to either reduce her allowance (including your additional costs), or to increase the time between dates.
Next, remember that the most successful negotiation strategies for SD's center around YOUR available budget (both funds budget and time budget) spend on her. It is NEVER about "her worth." She's "worth" a hell of a lot of money - just ask her! Telling her it's only worth (current amount less 20%) to fuck her will not end well. So start a discussion with her by saying that you really enjoy your time with her, blah, blah, blah, and you need to work through some budget issues that may impact your ability to keep meeting with her. Then just throw out some plausible reason (pay cut from work instead of layoffs, increased child support, extra travel needs for work, etc.). Just try to base your reason on some level of truth - lying will almost always blow up in your face, and not in a good way.
Then lay out two to three options for her to choose, like less ppm, less frequent meetings, cut out dinners & shopping, etc. None of the options has to be very good, but letting her choose one gives her the feeling she is controlling her life, not you. Of course, she may just call on you on your bullshit and dump you forthwith for an SD with a much bigger - er - wallet. Or she can reject all the options and throw it back to you. In that case, you need to either back off (and forever be her little bitch), or be prepared to end it (and be the manly man with blue balls that you know you are destined to be). "Well, I'm sorry we couldn't work this out. I really enjoyed our times together and I wish you well in your next adventures. I'll always be here if you want to reconnect"
Let us know how it goes, please. We all need to share so we can learn together.
Life is good
The Cat