I have always kept a sketchbook, though very tentatively before going to college when I thought it had to be filled with only my best sketches.Whilst there a tutor gave me some good advice which made a significant difference to my sketching, so I'll pass it on.
Its this....'Treat your sketchbook like a diary...don't let anyone see it if you don't want them to. Its yours to do what you like with, so it's up to you what goes in it and it doesn't matter what others think'.
ok pressure off.....its ok to mess up in it!
I have lots of sketchbooks. Some from my college days show all my failures working out illustrative designs and problems, as well as lots of notes about how to try things differently. They also contain a few poems or cartoon illustrations cut out from Sunday magazines....something funny to cheer me up when I got stressed or down. Thank goodness for Ian Dicks! (of 'Red Bull' fame)
My favorite sketchbooks to use are Seawhite black hardback ones in A4 which opens up to A3 size but also fits into my rucksack. In the studio I like to use the larger A3 size which gives me room to work larger and add photos if I want to.
I hope this section inspires you as an artist or student to keep a 'Sketchbook Diary' for yourself....I wouldn't be without mine.