Teeth Whitening with Carbamide Peroxide 15% and is it possible that this material is weakening my teeth?

I mean sensitivity for a long time, and a sudden need for unusual dental work right after (I had a need for a root canal over a fractured tooth and two holes developed at B side (outer surface) of two other teeth. I used bleaching procedure as instructed with a fitted custom tray, sleeping with it for a week. Should I replace this gel to be a 10% strength on my next bleaching (once every two years or so); or should I avoid this substance at all?

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