It's hard to give advice without seeing footage, but make sure you don't crouch cancel when you get Dsmashed.
Work on spacing and baiting, laser her to force her approach- dash attack if she dash dances too often and shield-grab her dash attacks.
Peach is difficult to edge-guard but edge-guards you easily so concentrate on stage control by maintaining centre-stage.
Build up percent and focus on Usmash and grab to Uair to kill her, always be conscious of when she's at death percent.
At the end of the day however I would say that Fox needs all his advanced tech to stand a chance against most of the top tiers as otherwise he's just combo-food. Hit the lab and keep trying it'll become second nature.
Being careful is incredibly important vs. Peach. Just practice patience as you're more likely to lose a stock if you make a mistake than visa-versa.
Watch some Mango vs. Armada videos to see the match-up at its best.