All arrangements end. Some end well, and others leave scorched earth in their wake. Most of time, you can control which path a "break up" follows. Most of the time.
It's fairly easy when she turns scammer, rob, wants to change the terms in way you don't like, or she stops performing as negotiated. Addressing scammers and ROB is obvious. Just block and move on. The other possible reasons can be addressed by calling out her behavior (keep it professional/civil) and asking her to explain. Then if appropriate, either respectfully end the arrangement or renegotiate the terms.
It's more nuanced when you want to change the terms or end the arrangement for reasons unrelated to her bad actions or performance. Here's some guiding principles to start and manage the break-up process when you want to (try to) keep an open path for future potential.
Just like your original negotiations, it's always about you, never her. The most straightforward method is to let her know you have a budget issue and it's just not feasible to keep seeing her. If done carefully, you can silently imply that you are open to a reduced allowance and/or schedule frequency and let her decide if she wants to accept that. Ideally, it's even better to let her tell you, rather than you asking her.
Do not say "I need to pay you less" or "I can only see you once a month". Just imply that your budget issue means you can't fulfil your part of the arrangement, and you would never ask her to take less (i.e.: never imply she's not "worth" your time and money). After all, you are a nice, upstanding guy who would never try to "cheap-out" on her. And the honorable thing to do here is free her up to pursue other opportunities.
Possible "budget" crisis: Lost your job, family member lost job or needs medical, your investments crashed, you got hit with business or tax fines, etc.
Another approach is to tell her you have a significant schedule change, like increased travel for work or family issues. If you won't be in town to see as often as promised, you won't be able to fulfil your side of the arrangement.
Ok, so that's money and schedule as "cause". One more is a change in your relationship status. If you have a wife or SO, tell the SB you really want to focus on your relationship because she deserves it, or you feel too guilty, etc. If not in a dedicated relationship, tell her you found someone (through traditional channels) and really want to give this a chance. (Can she really blame you for wanting "true love"?)
There are other approaches of course. But these may give you some ideas on what best works for you.
Final thought: It is important to avoid lying as much as possible. Lies will bite you in the ass eventually. It's increasingly difficult to maintain a lie over time. And women... well women NEVER forget details. Ever.
So try to base your justification on some level of truth and reality. (Ex; If you tell her you are moving out of state, it's inevitable that you will bump into her at a restaurant or shopping mall, etc.) If/when you end up connecting to her in the future and she asks about your situation, you can give her a seemingly honest update, because you are being honest.
Life is good
The Cat