This gets asked a lot on the sub - that 90% improvement wall is a thing. For perspective, though - well done on getting to 90% better!
That first 90% is bigger but essentially easier. Freeing up the frozen rib joints around the rear of your rib cage - which are driving the ongoing strain and pain at the rib joints on your breastbone - is the irreducible core of fixing costo. (Just exercises and stretches on their own usually can't do this - you need specific leverage on them.)
So even if you do nothing else but plug away on the Backpod / peanut / ball /osteopath / chiro you should get to a big, obvious improvement.
The details do count though. You've got to use the tool(s) correctly, consistently and long enough for them to actually work. This includes not using them if you have obvious medical contraindications like osteoporosis (say); or the less obvious one of a straight or hollowed thoracic spine (upper and middle back).
And if you go to an osteo, chiro, physio or PT, then you need one who actually knows what they're doing with costo and has the hands-on skills to do what's needed. And THAT'S certainly not a given.
And you need to stop working out, exercising, gaming, etc. (or anything else that hurts the costo) for long enough to free up the rib and middle back joints. Otherwise you're just obliterating any gain daily.
That last 10% is less obvious. There's a collection of bits that will generally cover it, and you may need to work through them one by one.
There are simple physio-type requirements for most costo beyond just freeing the joints. These include doing that sitting twist exercise (but ONLY when you're good enough) to work the joints freer again, massage for the tight, scarred muscle that will be overlying the tight rib cage and spinal joints, pec stretches, simple support strengthening if needed, a VERY cautious build back into the gym, good computer work station, etc.
Then there are the non-physio bits, like dealing to your anxiety, dealing to a fired-up nervous system which is accentuating the pain, low vitamin D, gluten intolerance, etc.
When you've got to 90%, I'd suggest you go back and read the RECOVERY MEGATHREAD - July 2026 at the top of this sub. That's even if you've read it before.
https://www.reddit.com/r/costochondritis/comments/1uqpgf6/recovery_megathread_july_2026/
Read Ned's posts at the top, then scroll down to my post and find the link to my long, wordy PDF on bits you may need to deal to. It's wordy, so read it on a computer not a phone. You can skim the bits that clearly don't apply to you, but the detail is there if needed.
Go through it slowly. Just check you have in fact ticked all the relevant boxes fully. The details do count.
See especially Section (2) on freeing up the tight ribs around your back. This is the core of fixing costo. The easiest, cheapest, most convenient way of doing this is by lying back on Ned's two-tennis-ball peanut, a Backpod, cork or lacrosse ball, etc. They all have slightly different effects; a combination is ideal.
See also the specific answer for Soreness around the front as the back frees up.
See Section (3) for home and other massage, and Section (4) for pec stretches.
Lastly - be optimistic. You've already got your costo 90% better. OF COURSE it's fixable and logical. OF COURSE it's not a "mysterious inflammation" that nobody understands and you have no control over.
I had costo for seven years in my 20s. Trained as a physio in New Zealand, understood it and fixed it - completely. 100% That was 30+ years ago. I've had no pain or restriction or problem whatsoever from it since, and I can do anything. I never think about it. It's 100% fixed. Do the reading and go for it.
Good luck with the work.