habitual bitcher
A person who's always mad at something. They complain like it's their job, and they take it very seriously.
'I'm mad at my coffee. It's not good enough. I'm going to complain about it for the rest of my life.'
'I got a bad grade. I'm mad. I'm going to complain about it for the rest of my life.'
'I'm mad at my life. I'm going to complain about it for the rest of my life.'