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It seems possible that your are going too fast. You say "go back" to Suzuki 2 and "your tutor may need to slow down".
Like I said in my mojo thread, I found my mojo again playing Dancla's easy and melodious stuff, for intonation and tone-control. You could get the viola version of that and transpose down an octave (it might be in IMSLP. There might be a lot of stuff in IMSLP you could use, for viola or cello. You were asking about books in another thread). And if you know it's your tone, just spend time at home on nothing but tone. It only has to be 10 minutes a day. Practise all your scales slowly for tone and intonation.
One possibility is that the music you play demands a lot from your left hand (I think you are already aware of this). That, my teacher's explanation has it, detracts from your brain's ability to control your right hand, and losing control of that may be your problem with tone. I assume you are not playing a left-handed instrument.
Going back to basics then is recommended by Suzuki, and it's not a bad idea.
Sounds like youre just getting a little frustrated. Hang in there youre probably about to make some head way on progress. But you know, even stepping back to something you thought you had a handle on might not be a bad idea either. It could be you just need a short break from practicing new stuff. Id most certainly share these thoughts youve posted with your instructor. He would be able to give better insight. Also I wouldnt dwell on the previous instructor..its water under the bridge now and doesnt help to ponder on what ifs. Dont beat yourself up!
My teacher always reminds me that learning is not a straightforward process. You never progress steadily higher all the time, but there are always points along the way where progress seems to stop.
I think it’s a good idea to slow down and even go back if you feel like it. I sometimes do exactly that if I feel that I'm lost with my bowing techniques. I go back to easy songs where I don't need to worry about shifting etc.
Another thing that my teacher always reminds me: open strings. So boring, but it is where the clear, non-scratchy sound is born. It’s so frustraring to go that level 0 when I have thousands other things I want to learn on my mind.
I know I'm expecting very much from myself. My progress has been extremely fast in this year, and I know it will not continue forever. I have to get used to it that sometimes I really need to slow down, do the basics correctly and carefully, so I can then reach even higher.
I've never laid a hand on a cello but I have lots of learning and instructional experience; what you're going through is typical and need not be a cause for despondency or alarm.
Much of the foregoing from the other respondents is sound: there will be times when you feel like you're losing what you've learned, and soon enough there will be a resurgence. We have an expression in the UK, "I've forgotten more than I know."
Happily, it's all still there in your synapses and once the current rest period has lapsed, you'll move on. Enjoy the simpler elements of your skill for the time being, and try to push the boundaries every so often, but not too frequently. Make good, but not so often.
"It is vain to do with more that which can be done with less" - William of Ockham
"A crown is merely a hat that lets the rain in" - Frederick the Great
My Bourrée I ended out the best I ever did after a few times through, and picking spots I noticed needed extra work. I then did it again to make sure it was not a fluke.
I then turned to my Bourrée II. This one is not as difficult bowing, or fingering, but does shift down the D string more, so notes I normally finger on the A string are almost all done on the D string. I love shifting! It was much much better, too. I went beyond the section where I was to stop, just to familiarize myself with that section. There is even more shifting on the D in this section, based on the fingering in the Bach Book. I wanted to see if I could figure it out correctly, so I penciled in string and finger notation, to compare to what my instructor says Friday when we continue with it. Sometimes he changes book notation, too. I didn't want to get too used to wrong fingering and string use, so, I didn't spend much time on it.
I’m starting to practice Bourree I and II by Bach too! I tried Bourree I and it’s not difficult at all, I think I will like it. I haven’t looked Bourree II so much yet, so can’t say anything about it. But in my book there is no fingering tips at all, so I have to figure them out myself and I’m little worried about it. 🤔
Bourrees from suite 3, same as you