I've started using my thumb again. After spending the last few months relearning how to play without using my left thumb at all, my teacher told me at my last lesson that it was time to start letting it "touch" against the neck once more. But I had to make sure that I kept it loose and not go back to squeezing it again; and to remember to let the hanging weight of my arm pull my fingers down onto the strings to stop the notes. Also, it is important not to forget how I've been holding my cello without my thumb (more pressure with the knees, etc.)
I realized that I had to come up with some sort of mental reminder to constantly assess what my thumb is doing - for example, each time I change strings or shift positions, mentally check to make sure that my thumb is loose and not pressing against the neck. [Another thing to learn to do while not thinking about doing it...]
The first thing I noticed was that I was able to play those sixteenth notes in the second part of the Vivaldi piece faster, cleaner and more accurately. (Although it was really hard not to let myself start squeezing again.)
Since that was my last lesson until July, my teacher left me with several new assignments. First, the next part of the Vivaldi piece - another Largo, in tenor clef and 12/8 time. Then, two more etudes in Percy Such targeting upper positions on the G and D strings. Also a piece by Rudolf Matz, Andante and Rondo for cello and piano - this one has some parts that go up into fifth position. Finally Suite Francais by Paul Bazelaire. I'm starting with the second part, "Chanson d'Alsace" - also in tenor clef.
It's so cool to be working on so many new things at once. Of course, I am trying to follow my regular study plan: [first clapping all the rhythms with the metronome and figuring out string crossings and shifts; then identifying all the other tricky parts. Next, after familiarizing myself with all this, I'll start playing pizzicato, slowly, measure-by-measure, until it begins to make sense. Finally, when my fingers "know" the piece, I'll take up my bow and go back and slowly work through each measure using both hands.]
(Writing this out each time helps remind me what doing.)
After trying out a set of Passione A & D strings for a few months, I got frustrated with a certain hollowness in the sound and ordered some Larsens from Cellos2Go. The improvement was welcome.