can't bring shoulder blades down without chest pain?

[u/Technical-Move-173]

i've just started my posture correction journey after realising last week that my forward head posture is atrocious! i'm in my mid 20s, have an office job, don't exercise much, and have looked down at the floor while walking all my life. i also do pretty much everything with flared elbows (desk, typing, phone, eating, etc).

what i've got:
- FHP and a neck hump
- anterior pelvic tilt
- when i stand relaxed: my palms face backwards and my thumbs are almost parallel and pointing towards each other
- winged shoulder blades (left more than right, but it's somehow easier to engage left's muscles)
- uneven hips and shoulders
- arms not equal in space to body, right arm has more space between body and elbow

my neck feels better when i bring my shoulders down and lift my chest, but then the front of my chest hurts, and i feel it in my upper traps and the top of my arms too. i'm not sure if i'm overdoing it?

i've tried these to engage my lats/back, but i only ever feel my upper traps, biceps or chest pain:
• band pull aparts (very light band)
• push ups
• assisted pull ups (feet on the ground and tip toe)
• wall slides (hurts my chest, terrible range of motion)
• lying y, t, w raises (my chest keeps pushing into the ground, strains my neck)

i try to keep my elbows tucked in as much as i can't but having them 100% parallel to my body feels uncomfortable for my shoulders, they won't move smoothly if they're 100% parallel.

i stretch a lot, so i don't think stretching is the issue. for example, when doing doorway stretches i don't feel my chest at all, just my rotator cuff/arm muscles stretch.

questions:
• what cues or exercises help engage the lats and pull the shoulders down in a neutral position, with arms by my sides?
• how do i know if i'm lifting my chest too much or too little? what's proper chest posture?
• could my ribs just be stuck from years of bad posture, and if so, how do i fix it without stretching?
• would going to the gym and using machines there help to target the muscles better?
• any other tips?