If she's already that weak, and there are maggots crawling out of her, inside her is 10 times worse.
Do you have access to a CO2 or Nitrogen tank? Is she totally listless or still some kick in her? Argon works too. So would nitrous TBH. You basically put her in a bag and fill it with inert gas, or a trash can or something that you know you inerted. Drop her in, a minute later she's stopped moving and is unconscious, 3 minutes later (i usually wait a bit longer) she's peacefully passed.
Chop the head off, Shoot her, gas her, you basically need to kill her at this point to put her out of her misery. THIS is also part of raising animals. it sucks but they expect it from you. although it cost money you could also take her to a vet to put her down as well.
Do you have friends, a farm nearby? get one of them to do it for you?
Aaron