I would recommend a chiropractor or a massage therapist first-- both are trained in techniques that can lengthen and strengthen the muscles and tendons that surround your temporomandibular joint. If it wasn't caused by surgery (many people's jaw problems are a result of too-early or just erroneous jaw surgery to 'fix' problems unrelated to the TMJ), either of these should work, and fairly quickly.
Often the problem is one of habitual, unconscious muscle-tension -- maybe not exactly a result of stress, but now made much worse by it. Sometimes it's a direct result of poor posture (head thrust forward, for one) and other times it's actually caused by uneven-length legs (the compensation for which carries all the way up the body and ends up with muscle tension in the neck and face). Sometimes, specific exercises can help, particularly in learning to identify and intentionally tense and relax the stressed muscles. Things like yoga, biofeedback, the Alexander technique and sometimes physiotherapy (although mostly I think those guys are just mean) may help.
I'm all for the non-surgical solutions, whenever there is one -- particularly when no one has any idea what the cause is. It's unlikely that surgery or braces will fix the problem any better than your wildly-overpriced retainer did. (I've never seen a retainer cost more than a grand!) All of these (surgery, braces and the retainer) look like 'treat the symptom' from here.