...that does not mean you need to tell her everything. Unless you think she is likely to up her game, lose weight, tone up, improve her dick skills, etc. with some constructive criticism you can just let her know it's been great and it's time to move on.
If she insists on some reason, tell her the truth, but tell her your truth:
"I'm not in a position to sustain my budgeted allowance for you and I wouldn't dare to ask you take less. "
Note: This is true! But here's the same statement before it was carefully edited:
"I'm not in a position to sustain my budgeted allowance for you (given you poor performance), and I wouldn't dare to ask you take less (because I want to spend my cash on a hotter, more skilled babe)."
So you are the kind, gentle guy who is facing the harsh realities of life (mainly her inability to give you wood - now THAT is HARSH!). This is the mature, adult way to let her out of her agreement so she can find a new guy (who may like/pay her more).
Tip: Never let the discussion stray into whether she is (still) worth the allowance. Every SB is "worth" a million dollars! Just ask her! Instead, focus on YOUR budget. She can't argue with your budget because she has no control over it. And if she actually does offer a 20%, 30%, 50%? reduction, you can decide if she's worth serving as a cheap fuck hole when you are bored or your new #1 SB isn't available.
Please let us know what happens... I think we can all learn from your process and experience.
Life is good
The Cat