I was defs pretentious and also quite self-pitying (but also, to be fair, clinically depressed). Wish I had kept a diary, it would've been good for me, and I've forgotten so much. I'm sure it would have been full of overwrought nonsense, but better out than in.
I did find one of my bleak teenage attempts at lyric-writing stuffed into my mother's toiletry bag once, which she'd obviously found somewhere. I only remember a bit about "The dopamine factory takes a 24 hr break" - it was fucking mortifying to think that she'd read it. We didn't really talk in our family, so if you wanted to know what was going on with anyone you had to count on finding bits of paper they'd scribbled their innermost thoughts on.
I started a diary a year ago. Partly to feel accountable to something in my bid to turn my life around. Weirdly, I'm only occasionally successful at being honest in it. I can write a whole slew of perfunctory "Did this then that" type entries before it occurs to me that maybe the thing that's been really bothering me the last few days might be worth a mention.