Haha. You're funny. But I know what you mean. The sad thing is: I've just quit handwriting things unless I have to for school. Just as you say, my handwriting is usually messy, I can never read my brilliant thoughts or recount what I was thinking at the time I wrote something, I always write too small, I can only use one notepad for one story (I can't just be schizophrenic and write different stories in the SAME writing pad, you know) so I always feel guilty that I'm wasting paper and whenever I need to look up something, I go straight to the computer and if I don't have the writing pad that I was writing my story in, I'll start typing it on the computer and then the story just becomes detached and I'll just move on to a new project and forget this handwritten one.
NO! I have made my decision and have stuck with the computer. That way, everything is neat and organized, there is no limit nor maximum of paper I'm using, I can write as fast as I please (almost as fast as my thoughts go). Yes, it makes me content.
Although, diaries... My parents always wanted me to start one, but ever since I did start one, it was things as you said that made me fussy. I once started a folder on my computer entitled "diary", but it just didn't work out. I think it might work for you, though, as long as you keep it up. Make it a special font or something. I heard there's a system on the computer that lets you make your handwriting the font (it might cost a bit, though, but for your sake, I hope you get it).
Hope your writing goes smoothly.