Thoracic spine problem

Sounds like your spine is a little stuck in its curves and can't quite stack yet. Stacksit only for very short spells, stretchsit and stretchlie your way to a straighter, longer spine and one day, stacksitting will work better for you. There are ways to accelerate this process, but whether you choose the slow route or the fast route, you need to begin with learning the basics.

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I think I have this same problem--stacksitting for an extended period of time tends to make my thoracic spinal area sore.  I do have a slight rounding there that I am working to correct with stretchlying and stretchsitting.  However, at work, sometimes I don't have any other option besides stacksitting (we only have stools with tiny backs to sit on when we are on the computeres)--what do you suggest I do then?  Is tallstanding a better option in this case or will I have the same problems?

In this case, I recommend you change your position often. Consider alternating between sitting and standing - and move around. In parallel, try to work on your posture as quickly as possible. First work on the basics, then look into mobilizing your thoracic area. We have techniques that work well that are taught in our advanced course. A few chiropractic visits can also help this process.

I also have rounding in my thoracic region, (why???) and it gets quite sore and feels almost tight when I stacksit for longer than a minute. I can deal with the discomfort if it means something good is happening, but I'm wondering if that discomfort is a sign that I shouldn't be going too much too soon?

I also exercise quite frequently, especially bodyweight exercises like push-ups, dips, inverted rows, and pull-ups. I have a strong "inner corset" and am very mindful while performing these exercises, nor do I feel any discomfort or that funny "numbness" sensation like I do while setting down trays at my waitressing job. Does this mean I can still continue to work out?

No, this kind of soreness doesn't serve a good purpose - avoid it. If setting down trays at your waitressing job is causing the problem, it's probably because you are rounding as you bend. Try hip-hinging with a friend or coach spotting you.

Sounds like your workout isn't causing any problems and it's certainly doing you some good.

Why we get rounded in the upper back? Usually because the lower back is swayed - and the lower back sways because the pelvis is tucked. The vertebrae are like building blocs in a tower. Each part affects the others.