This. I've been keeping a journal for about 15 years consistently, have probably 4-5 notebooks filled per year... and I'm pretty sure I would be needed to "decode" them should they ever fall into the wrong hands.
If they fell into the "wrong" hands, better to not decode them. Just make something up. :-) Going back and reading old journals (or old snail mail letters) can be interesting (or frustrating if the writing is too messy). I was reading an old journal the other day from when I was in my 20s. I wrote a line about how frustrating my affairs with T, D, and H were. I had no idea who T, D, and H were. I read through the whole journal looking for some reference to T, D, and H - and there was nothing. Eventually I figured it out. T. D. and H. stood for -- Tom, Dick, and Harry - in other words, a reference to men in general.
Yeah, write neatly AND explain your abbreviations so your biographers don't suffer too much.