Assessment context and image originals: https://www.reddit.com/r/AdvancedPosture/comments/14m7bg7/what_is_the_muscles_responsible_for_lifting_the/
Wheelie chair is a problem number one. But, since this is a still image I can pretend the wheels are locked / glued. It will work.
Your feet are place too forward to your plumb line and are too far apart. I also bet that your toes would be fanning out compared to your heels. It also looks to me like you have a fairly wide open knees. Sort of a wide stance / support.
Green line (plumb line) adjusted to the wall corner at the background. Placed by a big guess as to where the actual line should be. Without placing the feet properly first, I can’t be sure. But let’s go with this guess for now.
Purple spot – iliac (anterior superior iliac spine). That would need to move back and up in space until it hits the green line. This would be achieved by engaging the friction in your feet against the ground – you could transfer that load via your knees (knees going back) and onto your iliacs.
If that would happen, (and your chair is glued) your pelvis would rotate clockwise, at around your sitting bones. It would “curl / push” your sacrum down into the seat and it would have the tendency to push the beige spot (L3) back in space. It would start to straighten the red curve at the back.
However, It will not happen, unless other parts of the body will get involved as well. Let’s continue.
The green spot (bottom sternum) would have to be moved on the green line. Which in turn would shift the top sternum (blue spot) back in space, past the green line, so we would have to engage the top sternum to go forwards onto the green line.
The red spots are the two points on your 8th ribs about 4 inches apart, still on the frontal plane. You will easily find them by touch.
If you think of moving those two points together and back in space (as well as the green bottom sternum) to hit the green line, it will immensely help you with rotating the upper torso the way you want. That is, the ribcage needs to rotate counterclockwise.
Your chin would also need to go heaps forwards, and the black spot is your t1 (not visible on the image). If you would be to slide your left arm behind the head / neck and reach your T1 with your fingertips. (You will feel it, it sticks out and usually is where the collar of your tshirt is).
Then, you would ditch the phone (and get a tripod and record yourself) you could use your right arm to place your right thumb on the right red spot, and your right index finger onto the left red spot. That way you would have tactile feedback. Squeeze the two points together and push them back in space at the same time. These red spots are on the frontal plane, 8th ribs. About 4 inch apart. Always look for a bone sticking out a little.
While you do that, use the left arm on the T1 to bring the top of your torso forward. We are rotating the yellow line as the white arrows show.
When you attempt to do that, you will have the tendency to look down and drop through your front. Resist that urge. Head looks forwards at all times.
You will have to keep reminding yourself between the legs / lower torso and the upper torso / head. It will be very overwhelming at first, but certainly possible.