You're definitely not alone when it comes to feeling discomfort on the Tube! I'm not ordinarily prone to claustrophobia but in the past I've definitely had days where all I wanted to do was to get back out of the Tube and onto the street.
If you don't happen to live in a market where it's easy to avoid regional jets, definitely attempt to book a seat towards the front of the plane. I've heard from FA's and other pax that this helps. Even for experienced fliers, the long narrow fuselage can make for an unusual experience if you've never been on one of those planes before.
Personally, I get more anxious when I'm in far back in a 757 than in an RJ. Extremely long widebodies don't bother me - probably because there are enough bulkheads to eliminate the sensation of being in a long narrow tube.
Of course, if I'm at the very back of a 757 it probably means I'm on a Skyteam carrier - perhaps it's lack-of-status anxiety more than anything...
