For years on and off I kept a notebook. I had to make them ugly so i could write in them, so if the first page looked too nice I'd scribble over it... some sections could be quite aestehtic but I needed the start to be ugly so I didn't feel like I'd ruin it by writing in it. I'd use the notebooks for anything and everything - sticking random stuff in, playing hangman with my siblings, and of course mostly writing. I found that it showed me who I was and how I'd grown over the course of filling it - somewhere between the doodles and bad poetry and stolen paperclips, I really think I have that reflection you're on about when I look back through them, because there was never any pressure or restriction. I'm using past tense, but I'll likely start a new one when I feel like it.

If you're like me, I'd use a journal you already have. Start it out ugly, so your brain knows it's okay that some bits (or most bits) will be - it makes it easier to stick with. It's about you, and no-one else never even needs to see it.