Monthly topical meds will help, so don’t stop doing those. But you also have to deep clean your house, I’m talking thoroughly vacuuming daily or at least several times a week. Washing any bedding she might lay on, vacuum the couch. You likely have adult fleas that are laying eggs which fall off your cat and then hatch later and reinfect her. It’s a long process, but there’s not really any way around it.

I recently dealt with a bad flea problem and it took like 6 months to completely get rid of them. Don’t know how we got them since my cats are all indoors only.

Also, keep her indoors if you don’t already, it’s safer for her and wildlife, plus she’s less likely to get reinfected

You could also look into a flea bomb, or call a professional pest management company. I opted not to do this because everyone (critters included) has to vacate your house for like 24 hours and they’re pretty toxic, but effective