"Booy, I know this is not fair on my employer and I know I am being selfish. I don't want to be signed off at all - I want to go to work and do my job. But it is contributing to me being quite unwell."
yes but that means making a decision. you still have a responsibility to your employer to inform them of illness (depression is a mental illness) that will affect your work. this is affecting your work in such a way that means you cannot do your job. believe me i absoloutely get how you feel. i have been in your situation where i dreaded going back to work after being off but those feelings dont absolve you of your responsibility to your employer.
you need to make a decision. be that, to seek medical advice, quit work, change your hours, speak to your boss about reducing your duties in work, changing job, taking some time out. but you cannot do what you are thinking of doing. it is not a solution. it is a stalling tool. none of those choices may be appealing or even easy choices. each one of them may have an effect you dot want but sometimes there are no easy options. sometimes we only have a choice between two/three very unpleasant options.
in your shoes i would definitley seek a second medical opinion. i agree with you that you need to be out of work but i think you would feel far better about being off work if you got there by being properly diagnosed with whatever it is that's going on with you right now.
if you do decide to injure yourself and then you get close to being fully healed, all these feelings will come back. it isn't a soloution to your problem. you have to face this and deal with it. it wont go away by itself. work will still be there waiting for you after your foot (or whatever) has healed, but your MH issues will also still be there undealt with.