My experience (& I used to be good enough that I tried making a living teaching it, and I mean teaching it properly, not random self self-taught bloke passing on a load of bad habits to make a few quid...) is this...

I have, let me see... Two electrics, one classical; 3 lutes, one of them custom made (I was a re-enactor once...), two basses, a 23 string clarsach (wrong, wrong, wrong! It should have had fewer strings & they ought to have been gut...), a BIG rope tensioned side drum, a djembe, a wooden recorder (re-enactor, remember). Oh, and a set of (ancient now) Yamaha keyboards. I think that's everything...

In my experience, not just of musical instruments, the two real sticking points in learning any skill are the first 10% and the last 10%. Going from "I can't do anything" to "I can do something" can be incredibly frustrating. It's very easy to give up (I never have learnt to play that damn harp). The last 10% equally, it takes so much effort & dedication to go from pretty good to mastery...

The 80% in between? With a reasonable amount of dedication & effort, you can progress, but yes, Ake, there are plateaus. There are times when you feel you're not making any progress at all. What I've often found is that if you leave it alone & come back later, suddenly something has clicked, bedded in in the sub-conscious, whatever it might be, and suddenly... Off the plateau & rising again. Speaking from a purely musical p-o-v, since that seems most appropriate, that lick you can't quite get up to speed, that chord change you always seem to fumble... Relax, leave it alone, practise other stuff, enjoy your playing. Two weeks (or whatever) later, think "Oooo, let's have another go at..." And suddenly it's there, and you feel so much more positive. Suddenly it's "I'm so much better than I used to be" instead of "I'm not getting anywhere". Etcetera...