Originally Posted by Velo Dog

Could be both. The two aren't mutually exclusive...
To be honest, I only skimmed your post, but I've been in a similar situation and some of what I've learned may be helpful. Just some random thoughts...
--I'm about an inch taller than you, and a 58 would be WAY too small for me. I ride a 64 comfortably, and one of my bikes is a 65cm. Too late to change, if you need to, but be sure the saddle and bars are where they need to be. You're never going to be comfortable if they aren't..
--Where are the bars in relation to the seat? With back problems, you don't want the bars down low. Mine re level with the saddle, and I'm not lowering them.
--What size tires are you running? IMO, you (and I) are way too heavy for 23mm rubber. Grant Petersen at Rivendell has made a career of telling people over about 150 pounds to use fatter tires at lower pressure. that will tame the handling a little without slowing you down. I use 35 or 37mm tires at 75-85psi. Might not work for racing, but it's sure nice for riding around.
I suspect you'll get used to the handling. I swap among several bikes, from really laid back tourers to a fairly aggressive racer, and it takes just a few seconds to adapt (but I have thousands of miles on each bike, so that' could be a factor). if you decide to sell, though, try a frame in the 62-63cm range and see how it feels.