It was a lesson I should have learned when my IT was giving me issues: The location of pain isn't necessarily the source of it.
And my first clue should have been my teachers telling me that I "ought" to be able to do things that I was finding impossible, like navasana or tucking up into handstand. I was strong enough, they said; it must be mind over matter.
No, the first clue was that my lower back starting hurting.
The second clue should have been that the recommended remedies, such as forward folds, didn't do anything for relief. Neither did "tucking my pelvis" to take the arch out of my back.
At least I picked up on one clue: My slouchy posture was indicative of … something.
It wasn't until I randomly did a side stretch one day and found it to be a near-miracle cure that I realized that my lower back wasn't actually the problem. Then there was "pull up in your chest," which did a lot more for my posture than any amount of tucking.
I still don't know their name. Is it my obliques? My intercostals? My lats? Does it matter? I just know that I'm retraining a set of muscles that have somehow gone to sleep on me.
They're improving. They still have a long way to go; I still can't tuck up into handstand. But the difference is amazing - even if the amount of muscle gained is small, knowing where to focus my efforts has improved my handstand notably. It's also helped with other things along the way. The other day, I managed to get into lotus while in tripod headstand. It wasn't elegant, but doing it without falling over should earn me a cookie anyway.
It's exciting to have a breakthrough like that, even though I think, "Why didn't I know this ages ago?" Also exciting: No back pain.