Indoor cat has fleas please help

Both my indoor cat have fleas. I gave them flea medication last week and sprayed the house with flea spray, cleaned the bedding, vacuumed etc.

After that, I noticed throughout the week one of my cat itches a lot more in the same spot. Later I noticed some hair loss and a very red patch on her which was a rash.

I call the vets and we get an appointment, I take her in and they tell me she is allergic to fleas. He couldn't administer a strong flea medication because its too close to the time she last had it, and he told me it should cool down and wait a week.

I come home and check her fur and I see a flea moving! I call the vets and they say its okay and there's nothing I can do now and I should wait a week.

Currently I am vacuuming the house, unfortunately dont have any flea spray left (vets said its okay as I have flea sprayed recently) and I am changing my bedding etc.

I dont know what more I can do, please can someone give me advice? I used front line plus on both cats and now its been a week and they still have flea eggs/fleas. I feel so bad and unsure of what I can do further.

Im sorry I am very emotional right now, I know its only fleas, but with my cats anything makes me start crying and I feel like the worst owner ever. Please advise me of what I can do.