I do sort of the same thing, except half the time I can’t decipher it either.
I also do the Greek/Russian alphabet mix.
The closest I have to a secret code is cursive. Although, based on previous threads here, I imagine soon, it will be just my secret code.
This is my main form of secret code, especially since its the fastest. I have terrible writing, and years of deciphering it means I’m really good at reading other people’s chicken scratch. Cursive is harder.
In grade school I had a cipher based on parts of letters. I’m sure it could be deciphered in a minute, but it was convenient because I could just draw in the rest of the letter to read it.
I’m one of those super geeks who creates their own languages and alphabets to go with them, so I’ve used those for coding purposes before. It doesn’t work so well when the language doesn’t cover the same letters as English.
I use the Gregg shorthand squiggle for some frequently used terms (like ‘because’), the only ones I remember. I may throw some of the strokes for sounds, like the ‘ing’ ending. I’ve always used w/ for with, w/o for without. I will sometimes make a code for a certain person - after having a snoopy husband who read my notebooks. I also, like someone earlier, have keywords or phrases that speak volumes. And like many others, sometimes my writing is hard to decipher, especially if I want it to be.
In high school, I attempted to translate the tengwar (I think that one) from the Lord of the Rings, and used it sometimes. Years ago I found an interpretation of it on the internet, and it wasn’t quite the same as mine, but I don’t use it to communicate with anyone else anyway, so stuck with mine. I still have the key that I started with 40+ years ago, and sometimes take it and a calligraphy pen with me on trips as something to do on planes or at the airport, in an attempt to remember it. I think it can be obtained as a font even!
This. 
I often write things that only make sense if you’re thinking the way I was thinking when I wrote them. The act of writing something down helps me remember everything about where and when I was and what I was thinking when I wrote it down. There are so many notes around my computer and bedroom with short scraps of information like “use feudal Japnan!” and “in the regular world but with werewolves” and “like the Ayreon with Hansi”. There are also things that I write down and will only need for short periods, often stuff like “snake, eagle, fish” or “Iron, coal, diamond, malachite” or bits of chemical equations. Taken together, it would probably confuse the hell out of anyone.
You can try.
I use shorthand for learning tunes, Somewhat like this…
>I2 ^V2 _C2 ^V2 _C2 *L2 <B1 _C4
Symbols are Patterns, Letter is Intro Verse Chorus Lead Bridge, Number is times pattern is played.
I did exactly the same thing, right down to finding a slightly different interpretation online. (I never learned enough linguistics to understand exactly what Tolkien had intended, and it doesn’t cover quite the same range of sounds as English anyway.) I think I still have my original key and a journal that I wrote using it. I should go back and translate it, I think it was mostly records of a long road trip that would be fun to relive. There are several different tengwar fonts available here, by the way. Some of them are quite graceful.
I made up some shorthand for taking notes during my college lectures. Sometimes just the first couple letters of a word like photosynthesis. I’d use phs in my biology lecture notes. Then when I got back to my apartment I’d write out my study notes using the full words.
Even I couldn’t read those old lecture notes now. Heck, even a month or two later it would have been difficult remembering what abbreviations I used. That’s why I always transcribed my lecture notes to cleaner, better written study notes within a day or so of the class.
I never could write legibly balancing a spiral notebook on my knee. I always had to rewrite my notes cleanly after the lecture.
Ψου ηαϖε α σηιττψ σενσε οφ ηυμορ.
I still use the plain old business shorthand I learned years ago. I still remember it and how to write it, but sometimes it takes me two or three readings to decipher later what I wrote! There’s a very fine line between, say, the words ‘beer’, ‘bar’ and ‘bear’, and ‘bail’ in shorthand.
That is the problem with shorthand for long term note taking - if it isn’t fresh in your mind or in context, it’s sometimes unclear. Like, where you stuck without bail or without beer? Although neither is a good place to be.
Shorthand, hear, hear. In HS I formulated an elaborate code based on translating things into Russian (which I took for 3 years in HS) and then writing it out in shorthand. Thus rendering it meaningless to anyone who might encounter it, including me, a few years later.
Transliterating English into Cyrillic alphabet also happened, and I can still read that.
This is a plot point in Ellen Raskin’s The Westing Game, but with Polish rather than Russian.
Not secret enough for murder confessions or the like but close enough for me…
In high school I practiced long and hard to turn my handwriting into some mess that imitated the writing on the album cover of Pink Floyd’s The Wall (like on this CD).
I later married a Brazilian woman and learned to speak Portuguese in the process.
Whenever something … controversial … happens at work, and I think I should make detailed notes (e.g. a confrontation with some jerk), I write those notes in my regular work notebook, but in deep Pink Floydian Portuguese. That’s good enough to prevent random wandering eyes from figuring out what I was saying.
I read manuscripts for a living, and I take notes while I’m reading so I can write up my comments when I’m ready later. If I wrote them out all longhand, I’d have hand cramps and waste time I don’t have. My page for the current one reads C RS NI K TA TM R S TPM TO 3FL ACK M/T. I know what it means; nobody else has to.
My grocery list is similarly unintelligible.
A few years ago I was standing in line at the deli and there was a woman ahead of me holding her list. Of course I looked over her shoulder and she had written at the top “Grocery List.” That just cracked me up, that she had to put that. “I’ve found this piece of paper with items written on it: Swiss cheese, peanut butter, milk, aluminum foil, cat food, turkey burgers . . . what on earth could this strange . . . it’s as if it’s some kind of a list! Whatever could it mean?”*
*Yes, I know she could have written “Grocery List” on it and left it on the kitchen table at home for her family members to write down what they needed her to get. I just laugh at weird shit.