I was in a similar situation at that age, chronic unexplained upper/mid back pain. I got a job for a chiropractor that taught Neuromuuscular Reeducation (NMR) and he worked agreed to treat me. After just a few sessions, my knots and back pain were gone. His website is here: http://www.nmrseminars.com/ and he lists chiropractors and licensed massage providers all over the country who practice this.
The muscles between your shoulders run from your butt all the way to your neck. The best way I can describe this process is think of the muscles as toothpaste. If you want to get all the toothpaste out of a tube, you start at one end and squeeeeeeeeeeeeeeze it up to the other end. That's kind of what happens in this. Starting at one end of the muscle the provider applies pressure over the course of the whole muscle.
go to a website by Jolie Bookspan and read read read. sounds like you have WEAK midback and posterior scapular stabilizer muscles (espec rhomboids). this comes from poor posture-- ie rounded shoulders and upper back. what you need to do is strengthen the mid back and posterior scapular stabilizers and stretch the muscles in your neck (scalenes, scms, traps, etc) that are pulling your head forward. you need to get your head balanced over your spine.