Hi Cat,
I think what you're feeling is completely normal and very, very common.
When you have difficult conceiving not only is it stressful, but it starts to change the way you see yourself. We all have an idea in our heads, however vague, of how our lives will pan out and where we'll be at certain times in our lives. Having trouble TTC suddenly challenges those ideas and can, in my experience, bring on self doubt and cause you to question all sorts of aspects of your life.
I'm lucky, in that I love my career and it has been a source of pride and comfort during the last few years of unsucessfully TTC, but socially I have lost a lot of confidence and often feel isolated and excluded. I think it's all part of the same emotional response.
But..... I'm going to go against the tide here and say, however strong these feelings, be very cautious about making big changes while you're this emotional and while your going through the TTC process. You need to honestly and carefully examine your feelings and be certain of them before you do anything that can't be reversed. By all means investigate volunteer work in the sector you're interested in, but don't burn any bridges just yet - however strong the Monday morning urge to stride into your boss's office and shout "I QUIT" is!
Also, and I know this'll probably make me sound entirely materialistic and I'm honestly not, do think about your financial security before you do anything dramatic. Do you have the savings to retrain? Would you honestly be happy with a change in your lifestyle to take a more fulfilling job? What about when you do become pregant? Will you be giving up a good maternity package? NHS maternity conditions are good once you're working in the system, but it takes years to train as a nurse and as a student you'd only be entitled to state maternity benefit.
Littleshamrock makes an excellent point. Nursing is fulfillng and a noble profession, but it's hard, sometimes really really hard and generally underappreciated.
I'm not saying don't do it, I'm saying think long and hard and be honest with yourself about what you really want and what you're prepared to give up.
But most importantly, I'm saying you're normal, and what you're feeling is normal and I wish you all the best in acheiving what you want.