With the job you have right ow.. is it a job.. or is it a career?
I think if you've put a lot of time and effort and training into what you're doing now, surely you must have loads of transferable skills.. or are you talking about a complete change of a career.
I have a friend, who did engineering.. and is now training to become a physiotherapist.. so has to start right back at the beginning.
I understand your struggles, I'm not in quite the same situation as you.. but am constantly analysing what steps to take regardign my job situation..
I don't want to define myself by my job, but at the same time, if I'm unhappy in my job.. it affects my happiness in other parts of my life.