I am so sorry you have this major decision to make. The best words of advice that I received and use often are "Honor your limitations". For me, admitting that I will not be able to promise I can complete a task or project has been the hardest to face.
I agree with Dani, talk to your Boss, maybe between the 2 of you there will be a workable solution!
Sending you warm sunny (well er...actually burning hot) hugs from AZ!