Reddit reviews 170 Foundation Studies for Violoncello, Vol. 1

Ok. I'd definitely recommend buying a book of exercises to work through. In my experience, most technical playing improvement comes from exercises, not concert pieces. Without hearing you play, it's a bit hard to recommend a particular book. But you might try Alwin Schroeder's book. Just the first volume has a large number of exercises that span a wide difficulty range. Another book at roughly the same level is Sebastian Lee's 40 Melodic Exercises, which I just found out is available for free (!) here.

The key to these exercises is to progress slowly and methodically. Try to figure out the particular techniques each one is focused at, and be aware of that as you practice. Maybe set an initial goal of conquering one exercise per week (as they get harder, they will take more time). You don't have to do every exercise, but try to really master the ones you do work on. And don't skip the harder ones. ;)

One nice thing about exercises imo is that you can really see your progress. Six months in, you can count the pages of all the exercises you've worked through and you'll be playing stuff deep into the book that there's no way you could have played when you started.

If you need harder stuff we can talk, but I think those two books should get you going again. Don't forget scales and arpeggios; they really help keep your fingers limber.

Good luck!

practice etudes too. they’re beneficial like scales but they are more fun to play / sound a little prettier! here is a nice book of them if you don’t mind shelling out. it also may help to have occasional lessons to make sure your posture is still on point— makes a much bigger difference than you’d think

Feuillard (free on IMSLP is absolutely my bible. I don't use anything else any more for technique work. For etudes, everyone should have Popper - then, depending on your level, I would add Duport and/or Piatti collections. I spent years and years working my way through the Three Volume Schroeder Collection, it covers a great range of material. Probably 10-20% of the etudes there are a little thin, good to play through a few times but not worth real work. But I definitely became a good sightreader by just plowing through lots of it, and it contains gems from other collections (the aforementioned Duport and Piatti, for example).

There's also a very neat scales book called The Art of Scales. It's no Flesch or Galamian, but it has an interesting approach where each page is devoted to a key and you're presented with a variety of arpeggios, double stops, excerpts, etc for each key. It keeps things varied and covers lots of bases once you've already got your foundational work down.