It sounds like you need to work on your traps, lower back and (oddly enough) your abs. Luckily enough these are fairly east to build up by doing simple exercises daily but I'd recommend seeing a fitness instructor or booking an assesment at the gym before you start to make sure you don't strain anything or damage your back further.
I dive twin 12s with a 7l steel stage fairly regularly and do a nightly abs routine, two CV sessions and a weights session (chest, shoulders and triceps) a week to help make them more bareable!
