Set a 45-ish minute timer at work and get up and move for like 5-10minutes whenever it goes off. Try any number of the things everyone else had mentioned and see how it goes.
Next, take into consideration that part of posture is about managing your center of mass over whatever your base of support is at any given moment. If you’re goose-necking, it’s likely your center of mass (your ribcage, in the case of sitting) is too far forward. You need to get your weight
back. While sitting, that means over your sit bones. While standing, try closing your eyes and adjusting where you feel the weight in your feet so as to make it even front to back, side to side (in each foot) and foot to foot. Once you can make it as even as possible, just let your body relax. You might find that things auto-correct (at least to an extent).
As
@Symanoy mentioned, we ought to be able to move in and out of different postures. The issues only really arise when we are stuck in one. That being said, you may,
may be less stuck than you think….
Stretching and strengthening exercises are good but they’re only a band aid unless you can re-learn what efficient and relaxed posture feels like. Stretching and strengthening are tools, but they are one side of the coin, the other side being neuromuscular coordination and sensation. It doesn’t do us any good to do a battery of corrective exercises and then jump right back into the situation that made us need them in the first place.
Like you said, it feels “normal” to be in a posture you acknowledge to be uncomfortable or potentially problematic. Our brain gets used to feeling what we do a lot of as “normal.” So it is likewise normal to feel strange when you get to a “better” position if you haven’t been spending much time there. The main thing I would takeaway from this ramble is that posture should feel relaxed. If you have to squeeze any muscle tightly to have “good posture” you are defeating the purpose of correcting it.
Other “systems” I would look into include the Feldenkrais method and PRI. The former emphasizes being very relaxed, and for “movement correction” I have personally found that to be the number one most important factor. When you are at your desk, you need to find a way to sit that feels very easy and relaxed. Make sure your desk is a good height, that your screen isn’t too low or too far away, and that you can comfortably keep both feet flat on the floor. The last part might help with getting your weight back over your sit bones too. If you aren’t already aware, search for “typing posture” for a checklist of things that ought to help.
Hope it helps, and I’m happy to dig up other links if needed.
Here is a PRI-based trainer that has been making some good videos: