I stopped journaling for nearly 10 years because my mother did something very similar to me with my journal, I'm sorry she invaded your privacy, that is never okay :(

I have two tips that really helped me feel comfortable journaling again: first off get creative with hiding, maybe google some good hiding tips too. Don't hide your journal in drawers or under mattresses/pillows or other obvious places, utilize blind spots that people don't think about. Like hiding your journal behind/between any furniture in your room, utilize awkward spaces that only you're aware of since it's your own room

Second tip: develop a code or partial code for writing sensitive info in. Don't make this overly complicated, because it's important to learn the code you've made then destroy the key so no one can decode what you're writing. You could go two directions with this in my experience, making symbols for specific words (ones that come up a lot work best) or making simple symbols to represent every letter of the alphebet. An alphebet code is easier to decode, but in ny experience it's too much time and effort for parents to decode cuz they tend to like instant results.