being blamed for parents illness

my dad had a heart attack a couple of years back and now everytime my dad is in pain or feeling depresed my mum tells me its all my fault ,i dont know how to deal with this i feel its totally unfair...ive spent most of the past two years doing everything for him and now his suffering is supposedly all my fault...i cant handle this,can anyone advise

all the best
a very low
jc