It can be very tough taking care of your elderly parent(s)
because they're usually infirm as well. If you're also holding a
full-time job, the stresses combined can make you head towards a
breakdown. 
I guess there's no easy way to deal with nagging. I deal with it in
different ways, depending on my mood. Sometimes, I just 'zone out'
and practice selective deafness. 
Other times, I'll just
walk out of the room if I find that I'm reaching boiling point and
about to burst. Yet other times, I'll just playfully
exclaim, 'Aiyoh!! Talk so much! My eardrums are about to burst
already! Give your mouth a break, lah!'.
Caregivers have a very high possibility of getting stressed out.
That's why there's this service provided by various nursing homes
called 'Respite Care' whereby for a fixed period, the family
members can leave their elderly and sick at the home, while they
take a much needed break or vacation. This is a temporary
arrangement for a fixed period of time and when they return on
their vacation / break, they'll have to bring the elderly and sick
home again.
Sounds like you might be facing Caregiver Stress. Various hospitals
have support groups for caregivers like you. They meet regularly to
share their frustrations, share coping techniques, and make friends
with other people who are caught in similar situations. Find out
from your hospital / polyclinic if there's such a support group you
can turn to. Don't underestimate the role they can play -
sometimes, it really helps to talk about your frustrations, and
have a whole group of people empathise.
I certainly hope that you have help in taking care of your parents,
especially if you're holding a full-time job. It's not an easy
burden to shoulder, but I applaud you for choosing to take care of
them at home, despite the difficulties you're facing. 