I'm still a beginner but can say that my biggest mistake was taking years after my first year of playing then I came back and now play daily and love it. I probably would not have taken the years off if I had a teacher and engaged more here on the forum as I do now.
I also should have had a pop teacher from the start of lessons as I have little interest in classical studies so a lot of my practice was forcing myself to sit and work on it and I didn't enjoy the lessons much. Once I moved to a pop teacher few months ago I now get excited for the lessons and love what I am working towards.
I made the mistake of practicing scales one after the other in succession instead of learning on one scale at a time. And by learning I mean not just the scale itself but the relative minor, the chords, extensions, and inversions associated with that scale. I would've learned much quicker.
I'm still a beginner and happy to hear this as I very very slowly learn scales. Such as I dont just learn the notes and move on I drill the scale for weeks and months, HS, HT, parallel, contrary, chords, arpeggios, adding dynamics, etc. I didn't know if I wasting time but it sounds like this is a good thing to do.