Sorry to hear that.
Did this flair after your army service? How long did you serve- there is relevance to this question, don’t worry I’m not just making small talk.
For incredibly obvious reasons respiratory problems are a common cause of medical unsuitability to join.
Costochondritis can only be managed not cured.
The gamble for those making a decision isn’t an issue of permanent health problems, but the risk relapse. Anything that ends in “itis” means inflammation of some form. And inflammatory issues can always be re triggered and resurface.
Guidance says it can be aggravated by any activity that places stress on your chest area, such as strenuous exercise or even simple movements like reaching up above your head like rope climbing.
Generally speaking there's no shortage of similarly-placed blokes wishing to re-join & many are still perfectly fit to be soldiers, you aren’t. So your former service doesn’t mean a great deal.
Getting knocked-back at medical, for whatever reason, is a hard pill to swallow if you are trying to rejoin and I feel for you but be prepared to dust yourself off and look for a job elsewhere.
I once had the displeasure of having to deal with a self-induced casualty in the middle of a contact in a lovely little Kalay called Loy Mandeh (anyone else have the joy of this place?)
It later transpired that this lad, not long out of catters, had managed to slip through a net with a childhood respiratory issue. If anyone recalls the clusterf*ck it was during the op to turn one of the compounds into a TCP, the one only just across from CP Karim. Shudder.
Anyway this bloke compromised not only his own safety but his oppos! We ended up having to request a PEDRO (Too hot for MERT) to evac him following a 9 liner. Whilst this mong lay on his back wheezing and sputtering as our multiple dodged UGLs. So it’s a subject close to my heart. It is lessons like this that can’t be forgotten.
Regardless of your lovely MMA training in a nice gym, squaddies still operate in extrermes of climate and altitude, often without medical cover at the limits of physical endurarance.
Like I said good luck but I wouldn’t bet much money on you coming back.