When the project you scoped keeps growing past the price you set

I wrote the proposal, drew the lines around what we'd do, and put a price on it. Halfway through the project the client keeps asking for one more thing that's almost in scope, and I keep saying yes. The number I quoted made sense on paper and now feels too low for the hours we're pouring in. I can bound a job, mostly, but the edges always blur and the price never quite fits.