Thank you all for your words of support. I guess I'll give it at least until spring to see how much I improve over the winter. Then I'll be in a better position to decide. I've actually just started learning several more difficult pieces (levels 5 - 6) and I should have them learned by spring. Then I will have a larger repertoire to be disgusted with. 
Well you're not alone. I've been playing for a while, and the fundamental problem is that it never feels right. Whether that's playing something simple like CDEFG or something more complex. My right hand hurts after playing for a while too [which if nothing else tells me that I'm not doing something right]
As for pieces, it rarely seems worth learning the whole thing when the first few bars aren't correct, so I do a few bars of this, the intro to that, the first movement of something else and so on and like you, I've tended to stick with the same piece [or part of] long after memorising it to try and improve the performance of it.
Generally I find that I'd play, say, Minuet in G or a Clementi sonata as badly as, say, Beethoven's 49/2 or a Chopin Waltz. The latter takes longer to work through and perhaps I hit a bar I can't play, but the end result is basically the same in terms of how bad [or good] it sounds. Except perhaps bright sparks would say on hearing the Chopin to play something easier. But that sounds equally bad and doesn't make me any better at playing
...and TBH the Chopin is music I prefer.
So in that sense, I think you'll benefit a bit from playing some of those more difficult pieces, until you can find some way of learning what's missing from your playing, just to get some sense of progress and to learn something, even if it still lacks whatever is lacking from the easier pieces too.
I've tried lots of things - playing slowly, playing quickly, practising this way and that way, scales, moving the bench up / down, forwards and backwards. Mirroring my hands, playing just the right hand, or just the left. Recording the playing. Listening / watching others play the same piece. Concentrating intensely on the movements [from Bernhard's earlier posts] Forgetting the movements completely and just listening to the sound [from Bernhards later posts] - although the latter is difficult "live" mistakes and uneven playing stick out more if you record it [perhaps that's my biggest problem?] playing with a metronome or without and so on and so on.
Bernhard claims anyone can be a competent pianist. At the moment I'd beg to differ, and indeed youtube agrees with my experience afaict
I guess my conclusion is it's either of :-
(i) I'm never going to play well. Some people do, some people don't. or :-
(ii) Needing a good teacher - but IME they don't exist locally to me. I've tried some local teachers and they've generally been very poor. To be fair, they are good at pointing out what is wrong with the piece in obvious-to-me-already terms, I know the piece sounds crap and why in terms of tempo, evenness etc. They would probably be good at telling me stuff I knew before I started [Where middle C is, reading music, what a quaver is etc] but they are not good at all when it comes to offering anything practical or pragmatic to correct what's wrong
They might say "practise" of course which is true, but it's not that I don't practise. I'd accept a teacher who could actually say what was wrong with the way I practised / played and what is wrong rather than just the sound. I can post mp3s here and be told that much. They were generally ignorant about how to correct my playing so my arm doesn't hurt too.
(iii) Possibly being too harsh on myself, along with paying too much attention to teachers bragging about how quick their methods are with no evidence offered to back their claims. There are people who post the audition room who seem happy enough with their progress whereas I wouldn't be. Plenty of good pianists clearly took a lot longer / started a lot younger etc. So perhaps it just takes longer and a lot of those earlier pieces are supposed to sound a bit rough around the edges?
So I don't know, I guess I'm just carrying on because I'd hope (ii) is right, and a few fundamentals about playing notes on the keyboard comfortably would get me over the hill and because, even though I find the lack of progress frustrating and even depressing at time, I find the experience of playing, even badly, relaxing some of the time.
Mebbe one day I'll find it myself.
But one thing I will say is don't be persuaded to get a teacher unless that teacher is good, and that teacher has a bunch of adult students they've taught to a competent standard. i.e Only commit when you find a good teacher, don't think a teacher is better than nothing.
My experience says that's not true, I've taught myself pieces to the same standard, if not better than a bad teacher has easier pieces - anecdotal but I've done that experiement.
Topic: I'm going to be the world's lousiest pianist!