I find gratitude helpful, like Blaire. However the gratitude has to be spontaneous for me and not something I force and therefore become critical of myself for being "ungrateful".

I also find it helpful to honour my painful feelings / hurt fuelling my negativity (I'd have reacted badly to your mother's comment too), while also being curious about other perspectives. Like "I'm hurt by XYZ and yet also..." rather than "I MUST THINK POSITIVE"