As a trainer, I see and help people with this a lot. First thing is you need to find out what's causing it: is it stress (stress causes certain muscles to tense that pull your head forward), too much computer (or anything that puts your hands in the same position with a stooped neck, like reading, typing, sewing, driving, phones, etc etc), general slouching, low muscle tone, bad breathing mechanics, scoliosis or some other osteo pathology, or some other condition? Once you know that cause, you can start working on eliminating it. As you lessen the cause, you can strengthen the muscles that counteract what's pulling you into kyphosis. Exercises that do this include face pulls, rear delt flys, rows, pullups and quadruped arm swings. There are others, and you will need to look at your breathing mechanics as well. You're probably also going to need to stretch your pecs and shoulders and neck. To make sure you're getting everything lined up, you would do best to find an experienced and knowledgeable trainer and/or physical therapist. Your basic meathead might not be as concerned or educated in straightening you out. Also be ready to commit to this for at least a year. Progress with ligaments takes 6-18 months, so don't expect results in a week. You could see a chiropractor or a rolfer, but there are so many quacks out there that I couldn't recommend it. One visit to a chiro could fix certain joints if you can't get your head all the way back, but more than 1 is likely unnecessary. Find some trusted professionals with actual schooling and they should be able to help you out. Good luck!

Biggest thing I find, and I'm a massage therapist so 90% of what I talk about it posture, is find the little things that throw you out of whack.

Computer chairs are demons that clench your hips and back, fuck your abs, push on your elbows and shove your shoulders into your ears while you slouch forward and ruin your neck.

Standing desks are only as great as you are willing to move. Standing there, especially in bad shoes on hard floors, is almost as bad as sitting.

How you sleep effects your posture in the day. Make sure you've got a good pillow and your spine is aligned at night.

Final thoughts, STRETCH. STRETCHY STRETCH. STRETCH IT. STRETCH IT REAL GOOD. Nothing is going to make it harder to break out of that bad posture than tight, inflexible muscle holding you in place. Pec minor and your hip flexors are common problem areas people should check on.

Thoracic extension and rotation with foam roller

Shoulder dislocates with resistance band

Wall angels

Someone at work taught me this little trick for better posture:

Lift your shoulders as far up as you can.

Pull them back as far as you can.

Relax them back down, and try not to let them slide back forward as you walk/stand.

Great for a sore back.

Go see a Registered Massage Therapist. Most likely because of your prolonged poor posture, your muscle balance is out of whack and are compensating in all the wrong ways. Check if you get RMT coverage through insurance or work benefits, and they can tell you more.

I stopped carrying a backpack everywhere I went. Instead I put my computer in a much smaller messenger bag when I need to, and I switch which shoulder it goes over.

With the backpack, I had all sorts of shit in there just because one day I might need it. The backpack was constantly training my body to hunch forward.

Exercises that work the upper back. I like band pull aparts.

Stretching and strengthening in alignment to tolerance.

Willpower. I stand up, and walk straight, twice a day for an hour or so when I walk the dog. With perseverance, it becomes habit. Look at the posture of young people starting in the military — over time the discipline (of standing up straight) becomes natural.

You can get a strapped thing that pulls your shoulders back and forces good posture.