I've always been somewhat aware that my posture wasn't great, but I really started working on it when my knees got messed up and I had to physical therapy so that I could play sports again. Turns out that I got way over-developed quads after many years of walking, standing, running, and playing tennis with terrible posture. The quads (and hamstrings to a lower extent) would pull the kneecaps out of alignment and cause a lot of inflammation.
The physical therapist helped me figure out the proper posture for me. I had similar symptoms - rolling the shoulders forward, slouching, tilting head up, not engaging the abs. My advice was to push my shoulders back and then down, lift the top back of my head as if a string is pulling it as high as possible, tilt my chin down (waaay down. More. More.), puff out my chest, engage the abs. This advice is specifically to correct my posture. Your road to better posture could be different.
> Even right now, I'm studying and I notice I hunch over my desk, and my lower back will slouch. And tbh, it feels better/comfortable to slouch while I sit on my computer chair, than straighten out my lower back.
If you truly want to tackle this the right way, you will want to have an ergonomics specialist evaluate your posture when sitting. Depending on how your chair and your desk are set up, you can still have good posture in a more comfortable position. Sitting like you have a rod up your backside is not the be-all-end-all of proper posture and really only a specialist can tell you exactly what's best for you.
As others have mentioned, this is a looooong term project. Don't expect to fix this in a week, or a month, or even a year depending on your habits. Your back will be sore. You probably will slip back into old habits from time to time. But don't give up! Another thing that helped me is the Lumo Lift posture tracker. It is far from perfect and has some issues, but it serves as a great reminder for me whenever I drift out of good posture.
I'll be happy to answer any questions you may have.
If budget is of no concern, you could buy one of these.