Teeth whitening advice/help?

I want to whiten my teeth and i have the crest whitestrips supreme and a whitening tray with 35% carbamide peroxide. I read online that using the whitening tray can cause gum recession, is this true? Do the whitestrips cause gum recession also? I brushed hard while i was a kid so i do have gum recession, but i do not want to worsen them, but want white teeth also :/. Is it safe to use the whitestrips over the whitening trays (they come with the generic trays that you mold), and how would i go about preventing the tooth sensitivity caused by the whitening process? Thank you
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Brush harder, use mouth wash. Maybe.
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