Hi rangerkoala
First off, I want to let you know that my experience with doctors and the healthcare system has been pretty poor, so my response to you may reflect that. I know that there are lots of great medical practitioners out there, I've just had difficulty finding them.
With no medical background, I'm not even going to try to speculate what may be causing your dizzy spells. I'll emphasize, though, that psychosomatic issues are as real as physiological issues, regardless of what the doctors may say. Be calm and firm with your doctor that you need these dizzy spells resolved. If he/she isn't supportive, find a doctor who is. Easier said than done, I know. It's also easy for a specialist to look you over and say that you're fine from that perspective and brush you off as not in their field. Remember that each specialist originally had general medical training, so firmly ask for their opinion about what else could be causing your dizzy spells, and what your best next steps would be.
You sound as though you are in a panic, which is perfectly understandable, given the runaround you're getting from the medical community while your dizzy spells are totally disrupting your life, but panicking isn't going to help. You know this, so take this weekend to do nice things for yourself so that you can relax somewhat and think more clearly. Obviously, getting back to your job is a top priority, but you can't get back to work until your dizzy spells have been addressed, so that becomes your immediate priority.
Having dizzy spells doesn't necessarily mean that your depression has come back. There may well be a physiological cause that the doctors just haven't found yet. You've been doing well for 10 years -- what has changed in your life over the past several months besides the new medication? Does your chronic illness have an association, and if so, would it be helpful to talk to someone there? Other people with your condition may have also experienced dizzy spells and already found out what caused them.
If you haven't already done so, I'd also recommend that you tell your doctor of your job situation and financial worries -- he/she may be able to give you papers which qualify you for short-term disability payments of some sort. At the very least, telling him/her will emphasize the urgency of resolving this for you.
I've pretty much run out of suggestions -- just take a deep breath and don't jump to conclusions. You must be feeling pretty awful physically as well as being in the emotional panic, since dizzy spells are not fun at all. So be kind to yourself, take care of yourself, pamper yourself even, and don't accept "It's all in your head, and there's nothing we can do" as an appropriate response.
Good luck