Assessment context and image originals from this thread:
Here goes my 20 cents worth:
As you can see on the full body image, there is whole lot of things happening at the same time and they are all related. The way your posture currently is (as seen on the images) is due to the way you move the different parts of the body in relation to each other. These movements are unconscious (you aren't aware of them) and are habitual (that means you are became used to them - it feels normal).
Let's start bottom and go up.
Feet in a wrong position (the outsetps of your feet - blue lines) should be parallel. Your feet arches are completely collapsed. You can also notice that you habitually place your right foot in front of the left. It's on every single image and you will find that you place your feet every single time you stand and don't deliberately place your feet differently. You will always land like this - right foot forward. White lines at bottom for reference.
This can be also seen on your knees. Apart from the fact your knees (and ankles) are released, you will see the right knee ahead of the left.
There is a forward rotation with your pelvis (sacrum is moved too high up, iliacs are too forward and down).
You are rotating your ribcage back at the top (it's exactly the opposite rotation to your pelvis) - you can see that on the yellow sternum line (and the straight part of the red curve at the back). Your ribcage is literally between these two lines - rotated.
These two opposing rotations (pelvis one way, ribcage the other way) are resulting in the massive arch (shortening) in your back - red curve.
Your arms, you are pulling them too much back and up. Hence the shoulderblades sticking out at the back so much.
Your head is also heavily retracted backwards (although not seen). How do I know? It must be retracted back because of how the rest of the body is organised - gravity / equilibrium. Have a look at your original video, and you will see for yourself.
Apart from all this, you have some left / right asymmetries going on. You clearly carry most of your weight on your left leg (although it will not feel like that). You are tilting / rotating your pelvis sideways. The left right imbalance starting at your feet then compounds by the time you get to your arms / head.
There is a whole lot going on there. The shoulderblades / rounded shoulders was just a small part of the whole puzzle.
Hope that helps.