Out of my 20 years of playing violin, I had probably less than a total of 20 private lessons. Over the years, everything was in transition from very bad technique to trying to figure what works best. And in my first 2-3 years of playing, I had banana bowed. I had tight wrist on both left and right hands and couldn't really figure out good vibrato until my 4th year. But my intonation was decent. Once I was in grade 11, 5th year playing, I failed the high school All Region Orch and keep trying. Grade 12 (year 2008), I got in to TMEA All Region Orch, but failed to qualify to tape for All State Orchestra (total of 3 auditions for violins, [pass 2 blind auditions to tape] which was very competitive), and I was intimidated because I was sitting right next to the Texas All State rank #1 and rank #2 violinists who played the selected parts phenomenally/flawlessly. I was devastated even though I didn't mess up either! But what was different? What stood out? I ended up never going into Music Performance as a major and diverted.
What I did though was practice selected parts in concertos and listened to a LOT of masterclasses and legends (Perlman/Vengerov/Hahn/Heifetz...etc) playing and never exactly stopped violin playing, even though deep down maybe one day I would get better soon (how did they make it so easy?). I even want to give up completely and break the violin because I couldn't do a passage that I wanted to do. And the secret is: I have been practicing COMPLETELY WRONG. I was taking the passages WAY TOO FAST and was rushing to quickly learn the entire Concerto by sight reading and without looking at each note specifically.
So, coming to 2021, in February, I was able to learn the Paganini Cadenza by Sauret by self interest in 1 month time. And I never had a teacher coming to coach me. (Ray Chen playing this → https://www.youtube.com/watch?v=CKTRr4LH1gs&t=545s) I had previously heard Perlman and Yehudi Menhuin's flawless version of it on CDs many many times driving to work and school, and decided to give it a shot this year. I was successful even though I still have some inconsistencies here and there, but I qualified as a finalist in a recent competition I entered with that Cadenza.
There are times where I can't even start a piece by playing, but instead sit in bed to figure out the fingerings of the measure or notes to just backtrack if the finger pattern is even feasible for execution. Sometimes, in an etude, a fingering provided by the book is not suitable for that violinist due to finger dexterity, and adjustments may need to be made. If there's no other alternative that can be figured out, then the violinist must learn it that way, whether if the fingers end up being calloused, ripped, or stronger in the end, which is usually the case.
The biggest secret (I suppose I recent discovered) to successful playing is to take EACH INDIVIDUAL NOTE or REST and dissect it into its composer's intent.