Chronic lower to mid back pain - totally unresolved!

hey all,

So I thought I would try to make a post here to see if anyone can help, because so far noone really has been able to. Roughly 2 years ago, I started to experience some mild back pain, particularly when I was pack marching (in the military sense). Since then, it has progressed, I get pain when I sit or stand for more than say 20 mins. I also get pain if I arch my back. I could outline everything but it would take me some time. The details so far:

I'm 20
X ray - all ok
MRI - all ok (small but of osteoathrosis in L5/S1)
Bone scan - all good
Physiotherapist - unsure of why I'm getting pain (been seeing physios for 6 months now)
Blood test - all good
Sports specialist - unsure, maybe muscular
Doctors - ruled out things like arthritis and shermanns disease from blood test.

The pain is very unpredictable. Some nights I can't lay on my back, some days I can't walk properly, BUT some days I feel fine and can do most things. Consistently, however, things like long car trips ruin me.

I have some lumber/thorasic (spelling?) rehab exercises, most of which I find quiet easy, except the anything involving arching my back, which doesn't give a sharp pain, but a pain more localized to maybe a 3 inch area toward the top of the lumber. The cold also renders me useless.

Thanks

Can anyone help? I'm pretty much out of ideas :/ also, all scans have been double/triple checked by various specialists.