Wow, it's difficult to reply to all those comments at once but let me try

Pianistimo, Rach n Bach, piano121, imbetter, thank you for your kind comments! And special thanks to Pianistimo for your elaborate feedback and practice strategies.
I didn't expect any positive reactions on this, and I feared lots negative critique. Very happy it turned out this way.
Seriously, this really cheers me up

I really don't like not having any lessons:
Without a teacher, I have no idea what to do, I have no idea whether my playing is good enough or not, and I find it hard to estimate which pieces are at my level.
Pianistimo, as for your question,
you must have had a few lessons in the past? how many?
I never took any real lessons. My mother played piano when she was young. She has explained me how to read music, and other basic concepts (like what scales are), because I wanted to try playing piano. So I started playing piano and got a music theory book from the library which I am still reading (it's way above my level, and probably goes much too deep into all the concepts. As a consequence I now know for example everything about rhythm and metre, but almost nothing about chords and tonal harmony, because I didn't read that part yet

).
To be honest, I do also not know what a tetrachord is that you're talking about.
I could tell you a little more about my situation:
My practicing methods as they are now, are really primitive.
- I play no technical exercises.
- I warm up by practicing all the major scales, all with the same 12312345 fingering (not good, I know. but the transition to the 'modern' fingering is so difficult

). First LH, then RH, then HT parallel upwards/downwards, then HT going opposite directions.
- After that I start to practice the pieces. I can only learn them very slowly, because reading sheet music is more like deciphering sheet music for me. My main strategy is to record and listen if everything sounds good (recording is very easy on a digital

). In the 2 years that I play piano I have only learned
Bach C major prelude WTC book 1,
Beethoven Fur Elise,
Bach Invention 1,
Bach Invention 8,
And this Mozart K331 movement.
That's all really. I assume this is not the way to go: I feel that I'm progressing really, really slow.
The time I took for every piece, especially this one, is frustratingly long.
So I'll be trying to take all your advices into account. I will be starting to work on another piece.
I think the 2 inventions I learned are the pieces that have taught me the most in a relatively short amount of time.
So I'm thinking about learning another invention, don't know which one but probably one with lots of trills, because trills are a huge problem for me.
I will also take a look at some technical exercises, probably these 'a dozen a day' exercises (there's no way I will handle Czerny's finger dexterity

)
Again, thanks a lot for the help! as you noticed I really needed some guidance (and probably still need it, so still trying to get money for lessons

).