Original request: https://www.reddit.com/r/posturepals/comments/1b0w7tr/how_do_i_fix_this_posture/
Permanently fixing this posture would require that you learn to coordinate (move) the different parts of your body in a new way.
First you would have to learn to understand what is going on with your posture currently. Then you would also need to learn what correct posture looks like and why it is "correct". Next, you will need a method, or better to say technique that will help you to get from the current coordination to the new and better one.
These images aren't great to do a full assessment, but there are issues visible as is. I can comment on those.
So the red curve is your spine. Specifically in the middle of the torso. You can notice the distinct arch, which we call "shortening of your torso". It happens for two reasons.
First, you are rotating your pelvis forward and down at its top. Given the centre of rotation being in the hip joint, your sacrum is lifting up.
Second, your ribcage is also rotating in space. Except this time in the opposite direction to your pelvis. That is the top of the torso goes back and the bottom of it is pushed forwards. You can see it on the slope of your sternum (yellow line). Your ribcage is right inbetween the yellow line and the top part of the red curve (your spine).
Because you have these two opposing rotations of pelvis and ribcage, the middle part of the torso, where there is just the spine and your abdominal cavity adopts the shape of a curve as you can see marked.
On the image with the Tshirt, you can nicely see how you are retracting your entire arm assembly too far back. Bringing shoulderblades far too close to each other (pink arrows) and lifting shoulders up. Elbows too are pointing backwards.
Many more issues involved, but not visible on these images.
You could consider taking better images: https://goodposture.studio/how-to-take-images-for-posture-assessment and I can explain the rest of it.
However understanding the above text would be first step in learning how to permanently change your posture.
The Initial Alexander Technique (IAT) was developed exactly for this purpose: how to permanently change ones posture (and ideally for the better). So IAT is what you want to learn.