Here’s a quick and simple way to check your posture from the side. Get close to neutral. Be there as long and as often as you can.
There’s a ton to unpack here; but I basically see most people hold the common position, a severely amplified version of the anterior pelvic tilt. The back is extremely hyperextended/compressed; the abs are extremely lengthened; the hip flexors are supremely short; the hip extensors are long and dead.
There’s no “quick fix.” Become aware. While watching in a mirror from the side, see if you can even get near neutral. Most people can’t, let alone a flexed spine with short abs, contracted glutes, and extended hip. Regularly, I’ll have to have people do a modified lying bridge, a kneeling “cat back” or a wall stand for tactile cues, and they still can’t get near neutral. Not even close.
It’s going to take time. So just first become aware. Start there. With even halfway decent control and mobility, the front of the pelvis ought to be able to face upward with tailbone tucked way under. Most people cannot even get the front of the pelvis to face forward.
I’ve worked with clients who start out with pelvis facing directly down into the ground and wondering, “why does my back hurt all the time?” They’ve learned momentary relief from forward folds and other movements to temporarily decompress the spine; but they still have zero control over glutes, abs, and pelvic positioning (and I won’t get into femoral rotation and a variety of other pertinent layers right now) - so nothing fundamentally changes. Oftentimes, those brief “fixes” actually amplify and worsen the real long-term problem.
It’s complex. Just start with awareness. Work TOWARD control such that you remind the body neutral even exists.