decision time: work

It got dark early (DST) today...it's going to take some getting used to. Not that it was bright out very late anymore at any rate...
I need to make a decision about whether or not I can finish up the work my client gave me. They contacted me for an update. I've been putting things off, but need to at least let them know if I can finish the work I've got in my queue.
The same old arguments apply: I know I'm still depressed. The thought of going back to work still fills me with anxiety and a bit of fear - but I have to admit that it's not as bad as it was... I feel a lot of pressure from myself to finish up the work since if I don't, I don't know where else I'll be finding work in the near future and money is definitely tight. I don't know what to do. I feel sort of sick thinking about it. 
My fiance has been very supportive and let me know that if I can't do the work, then I can't do the work. He hasn't pressured me to take it. That said, he works so hard, and money is tight. I feel like I should take the work and be grateful that I have any to begin with.
I'm still stuck in that place where I don't think I can do it, but now I need to answer the client. I feel like I will be letting them down. I feel like it is unprofessional to turn the work back. I don't know what to do. I wish I were feeling well enough that I were looking forward to doing the work. I wish I were well.