Cured for idk like over a year now
Imma keep it simple cause you've seen it here already. This condition is very aggravating and I had it for years and couldn't sleep on my side at all with how bad it was.
First and foremost - backpod (you can go for an alternative?? Idk. It ain't that expensive and it's well worth it. There's a reason everyone recommends it. Alternatives might work. Do you.)
Do that until you can easily do it with no pillows as per the instructions.
Then train again... Slowly. Very slowly. Like literally pick up a couple five pound weights and do chest flys. I personally only did chest flys.
You can do more if you want.
But go slow and do low weight. Progress up in weight slowly over time. I try to wait til I can do 3 sets of 8-12 reps with pain less than a 4/10 (looking up pain scale helps, cause pain is normal and acceptable during recovery while at or under a 4/10)
I can chest fly 30lbs now and my chest no longer is in pain.
***Most important part***
Yes it will hurt during, and after, working out. Yes it will feel like it is getting worse.
Do backpod again, wait a day (or two), and do it again. Trust in the process. You have to strengthen things, otherwise you'll never get over this. I'm convinced of that. Stop if the pain is greater than a 4/10, and try again in a couple days. Rinse and repeat.