Sorry to hear it was a bit of rough spellwork for you, @NoName. Perhaps the flames of the candle were reacting to your emotions? If you were feeling stressed, it’s possible that that tension may have carried over into your casting
As for hiding your journal entry, I’ll tag @Francisco who will likely have more spell-specific reasoning for you, but if I can add in my two cents, I think you can still use the notebook. You could tear out that page to hide it, or cover it with another piece of paper, or just try not to flip to that page (you could secure it closed with a small piece of tape or something).
As for why some spells require hiding our written goals, I think it could be for the same reasons why we often hide away treasures and valuables. This is something very special, so to handle it with care and tuck it away somewhere safe may help to protect the idea. You could also think of it like a seed Plant it away with care so that it can sprout and grow in safety
Just my thoughts about it! Personally, I like to use manifestation spells with fire - I like the idea of burning the wish to set it free, sending it out into the universe to manifest. But witches who prefer to use fire magick for banishment may see this as banishing the wish, and prefer wish manifestation spells that preserve the written wish to keep it safe
I always recommend experimenting with different types and styles of spellwork to see what clicks best with you!