I’ve also dealt with this. Try exercises to strengthen your core, lay on a foam roller or the floor to stretch your back out, use a resistance band or gym equipment to target the muscles in between your shoulder blades, stretch your chest, shoulder, and neck muscles, and do some light exercises to strengthen the back of your neck too. All I do for that is just lay on the bed with my head over the side and lift my head up and down until I feel some muscle fatigue. You can also lay on the bed on your back and lay an arm out over the side to stretch your chest muscles (or you can do this on the roller). I’m not a medical specialist by any means, but I’ve found doing these activities to be helpful for my rounded shoulders. Strengthening the area in between the shoulder blades has helped me to keep them back with less effort. I find a good spot for the resistance band in the gym where I can target this area, but the gym equipment has also helped too. My physical therapist kept telling me to get a gym membership. I took her advice and it has helped.