Notes

Wanna develop awesome, subconscious effortless posture? Crawl like a baby. Keep your head up, chest up to look at the horizon and crawl. Try rocking forward and backwards like a baby does too. Rock forward and back 10x, then hold the back position and nod your head 10x. Rocking will reteach your pelvic floor to REFLEXIVELY (not consciously) engage which is the root of good posture. Don't stretch this and strengthen that. Practice "primitive" movement patterns (rolls, rock, nod your head, crawl) and your posture, diaphragm breathing and reflexive core strength will return in no time.
You don't even have to do it for a long time. Try rocking 10x, nod your head 10x, then classical baby crawl for 1-2 minutes (on the tops of feet, head up, chest up, look at the horizon! thats important). then stand up and see how it feels. If you like it, try doing that 2-3x a day. This will stimulate your vestibular system, which is important for posture as well. Head control is one of the first things a baby learns. Moving your head around in a full range is really important as the body follows the head.