Says Jen:

Just as I thought I was beginning to get a handle on playing with complete relaxation……….I tried the thumb position. My left shoulder seems to lag behind in this shift. I spent the majority of my time telling myself to drop my shoulders. If this were football, they would call it “alligator arms”- playing without my arms/ trying to use my hands only.
I tried to get a little more adventurous today and practice my piece with the slurs intact and found that as soon as my bow hand was asked to do something more difficult the tension reappeared. It became somewhat of a roadblock so I worked some more on scales and avoided the piece. It was disappointing that I couldn’t progress further but the experience of just playing scales without any tension is so enjoyable that by the end of the session I felt pretty good. I can’t express enough how much more in control of the instrument I feel and how calming it is to play this way.

I like your first sentence! It’s close to, “Just as I was beginning to develop the Grand Unified Theory…” and “Just as I was beginning to solve the budget crisis…” You are doing something that amounts to a life’s work, so don’t be surprised if your body deserts you from time to time and you fall into old habits. Being on top of tension is like being around a 2 year old in Forever 21 (something I did this weekend). You have to constantly be looking out for disaster, react quickly with relaxed confidence, and never be surprised at how persistent the will is. Habit is strong, but you can build a stronger habit of awareness and gentle correction.

Undoing years of tension can be likened to unraveling a ball of yarn. It should just be pull, pull, pull, and you end up with a long piece of yarn. Instead, because we’re always in a hurry, there are knots that take a long time just to discover, let alone pick apart. You have spent only about 48 hours with some kind of control over one aspect of tension. Give yourself 48 more to dismantle this new part.

Thumb position is easy to freak out on. The way I used to remind myself to stay cool and collected was playing “primitive cello”. I would bring my hand up so relaxed that I could not play a note. Imagine how you’d play on sleeping pills. I would just let it sit on the fingerboard; heavy, inert, collapsed. When I had checked up and down my arm for thorough laziness, I would roll back and forth from my thumb to my pinky, keeping all fingers loosely in contact with the strings, but not pressing to make contact with the fingerboard. Then I would lightly pulse my index finger up and down, making marginal contact. The hand should look stupid and not like a cellist’s. I would pulse 2 and 3 after I had warmed up with 1. Oh, and none of this is with the bow. This exercise begins in silence, except maybe the sound of drooling (if any). The key is to see that the fingers move best without interference from a tightness in the hand or wrist. Then play a primitive Mary Had a Little Lamb. The less accurate the notes are, the better, because you can’t take yourself too seriously and hope for no tenson. If your fingers collapse (knuckle down, not up) then even better. Breathe through your mouth and make that Teenager Watching TV face. Total relaxation, boredom, now go pick up your socks and is your homework done? Just kidding. But you see what I’m driving at. You have to come from a place of extreme looseness rather than try to recover and backpedal from existing tension.

Last, you know that it’s always the bow’s fault. Or at least 85% of the time, it is. So control it, and apply a little “primitive” to it, too. Aside from avoiding the usual pitfalls like speeding through an up bow or death grip thumb, try taking a teeny amount of bow and resigning to a grainy sound. Remember, when you’re working left hand, the right has amnesty, and vice versa. Don’t beat up the hand you’re not working on. It’s pointless and has a way of crapping out the entire gesture. Revel in the feeling of relaxation, and just make a note that as soon as you feel a little better about thumb position, you’ll clinic your bow with long, boring strokes that produce a tone befitting such a marvelous left hand.

You have not taken a step back at all. This next one is just a sort of StairMaster, and then it’s onto the next.