Psychological back pain?

I never came close to reaching an international level as an athlete. The main reason was lack of talent, physical and psychological. Throw in a large portion of laziness and lack of self discipline, and that puts me where I am today: a middle aged man at the back-of-the-pack-at-small-local-club-races level. But I thoroughly enjoy it, nevertheless.

Cross country skiing should probably be my strongest sport, but pain in my lower back has made it difficult for me to ski at full force. I remember feeling this pain occasionally during my late teens, but since I reached thirty it has been getting worse year by year. This pain always starts in my lower back and then leads down through the hips and to the thighs. It makes each stride painful, and it is almost impossible for me to stay in the "hockey" position on the downhills.  Sometimes the pain wears off after 10-15 kilometers, sometimes it lingers until I finish. The funny thing is, that when I stop it only takes a minute or two before the pain disappears, and I never feel it in my daily life or in the other sports I engage in.

I have been to physiotherapists, doctors, specialists; I have been massaged, x-rayed and researched thoroughly but I never get any better. Actually, no one can find anything wrong with my back. One specialist told me that my back was exceptionally "well built". But even that doesn't make the pain any more bearable.

A few weeks ago I heard an interesting theory. There was this guy who told me that my pain was probably psychological. He believes that, however it started, my brain now recons that skiing and back pain should go hand in hand. The brain, therefore, simply makes the lower back, hips and thighs believe that they are suffering whenever I go skiing. The other possibility, he said, is that the pain is some kind of a defensive reaction, so when I know that I´m going to be tired, struggling and suffering out on the course, I simply imagine this back pain to give myself an excuse to slow down and relax. Honestly, the pain feels real enough, but after so many unsuccessful attempts at understanding it and getting rid of it, I'm not closing the door on any possible explanation. But how do you make an imagined pain go away?