Have a look at a Curble chair, it's not a full chair, but can be placed on a chair, sofa, bed or the floor and its shape automatically straightens your back and shoulders.
My dd has hypotonia where the muscles don't attach to the bones properly and that makes all her joints floppy. She tends to slump forwards in the middle around the diaphragm which then gives a hunched back, rounded shoulders and a jaw that juts forward.
This chair has helped a lot, as have several other things. Pilates for core muscle strength, Alexander Technique for being aware of how you move. Actors use it a lot as do dressage riders for posture control. Some back braces do train the muscles to be in the correct position, but be aware that because they have been incorrectly positioned for a long time, this feels comfy and the correct position feels uncomfortable, so it's hard work to keep doing something that doesn't feel good.
The OT we had when she was a child said that when sitting, the hips, knees and ankle joints need to be at 90 degrees, no slouching.
There are loads of helpful youtube videos, the 'trick' is to persevere when it's uncomfortable.
I'd recommend a few lessons with a practitioner simply because what feels like correct posture to you may actually not be and they can save you a lot of time and effort trying to get it right on your own.