I won't say "I don't want to do it or I can't do it"

. "I don't want to do it" shows bad attitude, "I can't do it" shows lack of confidence or capacity.

Always say "I will do my best to help", but do let them know if the assignment is not feasible/realistic. If t it NOT your responsibility to achieve something that may not be feasible, but it is your repsonsibility to let them know (that is not feasible).

then do your best, and after that, whether or not the project gets done on time and on budget with high quality is management's problem, not yours.