Registered just to reply to this, as this is one topic where I can say something intelligent.
Yes it can be reversed.
First, the thing where the posture got really out of alignment from sleeping with the neck forward, that is something a chiropractor should look at. Find a good one who specializes in the upper back, but not "upper cervical" - that is a very narrow specialization that is quite different and may not be what you need.
The chiropractor will also be able to break up what is effectively scar tissue around your vertebra that holds the forward slouched posture in place and congealed. With age in a lot of people the vertebra actually fuse together and cause very significant pain, definitely something to be avoided. The body will simply dissolve the scar tissue once it's broken up. That's how I understood it anyway. Doesn't take long to fix, only a few sessions.
Parallel with that, do exercises that strengthen the muscles that hold the correct posture. Many of us have tried to hold a good posture consciously and we know how well that works. Try to walk in a park for one hour while holding good posture consciously. Not only is it impossible to focus on that for one hour, but the muscles of the upper back are stretched out and weak from computer work or similar, so it's very difficult to do.
So lie down on a bench face down, extend the arms to the sides, and lift them up backwards and hold for three seconds. Try to do twenty reps, or two sets of fifteen. The first time I did that, without any weights, after like ten reps the arms were shaking, the upper back was just comically weak, and I wasn't weak overall, doing bodyweight training, convict conditioning.
Once twenty reps or two sets of fifteen become easy, add some weight, one pound in each hand, even just a water bottle will do. After two and a half years I do it with five pound weight in each hand for 16-17 reps working towards 20, a far cry from where I started. Also do the same thing with the arms extended forward.
There are several other exercises with the same basic idea. One is called prone cobra. This guy's channel has a number of them, some more useful that others, but basically just do the ones that are difficult for you.
You can also get a head weight, like this one a company out of Jasper GA sells:
I actually use two of those together, for a total of eight pounds of extra weight on my head. If you put the weight on the forehead, that works the back of the neck muscles. If you put it towards the back, it works the front of the neck muscles, which are in fact the ones that help to hold the correct posture. That may be counter-intuitive, but if you think about it, it makes sense.
Also you can get a resistance thing like this one:
Pull it as you move your arms backwards in any number of ways, same idea as the bench exercises.
Once the right muscles get stronger, the default posture becomes much more upright. And if you try that one hour walk again, you'll find that you can just set an exaggeratedly upright posture at the start and forget about it, the body will just hold it for a full hour, because it is now much easier.
