Please don’t stress yourself out.
I know better then most people how easy it is to be so caught up in finishing a costume that you start sacrificing your mental or even physical health for it. But remember this:
Cosplay is a hobby. A hobby is supposed to be fun.
Yes, there are exceptions to this, for people doing commissions or who make their money at conventions. But the vast majority of cosplayers do it as a hobby. Now, this isn’t saying you shouldn’t cosplay if it stresses you out even a bit, but the week before a convention isn’t supposed to feel like it’s going to kill you. No one should have their health effected by a hobby.
If you’re losing sleep from working 24/7, if you have a panic attack thinking about what you need to get done, if you get sick from working so hard?
Stop. Take a step back. Relax.
No costume is worth your health. It’s alright to back out of a costume if you find out you can’t do it. Yes, it sucks to have to drop out of a group or tell someone you aren’t doing a costume, but your health is more important then cosplay.