The history of back pain for 15 yrs is troubling in a young person. Poor posture alone would not result in such a lengthy and problematic course of treatment.
You did not mention what diagnostic tests have been done, such as x-rays or MRIs.
All of your exercise programs such as yoga and swimming would normally produce more lasting results.
If no one has done any tests, it would be best to request further investigation by your primary care doctor and hopefully a referral to an orthopedic spine specialist.
I hope you are able to find a physician who is willing to perform the appropriate diagnostic tests.
Best wishes ----
I hope your problem is resolved by now. I just read your post and it sounds so much like what I have been experiencing for about 8 years now. I am a very active 58 y/o male. In about 2003 I was running a marathon and started having upper back pain , just as you described, in that very spot. Long story short...I have a herniated disc @ T-7. This is very unusual from what I understand. It is very painful, every day painful. I am under pain management and it helps some. I have had several rounds of physical therapy to no avail. Have had "nerve burns" and other things with just slight relief. They say surgery is not really an option. Too risky Taking pain meds for about the last 3 years but it is just a temporary solution to a long term problem.....If you find any answers please let me know. Thanks