This deserves a much more thought out and nuanced answer than I am capable to give.
I will try anyway.
Let's take something that we have more information about: burnout. Since burnout is a hot button topic, we're all somewhat aware about it.
Many people misconstrue burnout to mean "overworked" - which it's not, it's a type of depression where your emotional investment is not getting adequate emotional returns: and that's what's happening with your depiction of "bitter".
You had objectively worse situations happening to you, yes! However- the conditions in which they happened were:
* Not artificial. There was no concerted effort by the universe to conspire to give your fathers cancer.
* You were given sympathy
* You were given the opportunity to actually air grievances about it before it boiled up- likely you were told that it's healthy to feel bad or to express yourself.
Likewise, bitterness is the culmination of being treated in a way you perceive as unfair, and it starts small. It gets worse when not treated. Treatment is as easy as letting people be a little angry sometimes or to let them talk about their issues and be met with something other than condescension.
You had a worse situation, yes, but you're talking about people getting moody as a moral failing.
It would be like me telling a woman not to be moody on her period because some men have their arms blown off on oil rigs. They're not comparable at all.