Join the club! My mother is similarly a Polyanna. And the counterpoint tune is no better, when she becomes an amateur psychiatrist and diagnoses depression and recommends drugs in one foul swipe. The lack of empathy is so negating. No point telling her how you feel, since you know what the response will be.
On the other hand, we must learn how to develop good feelings in ourselves. It's something I've had to continually work at and does not come easily for the most part.
Sorry you are in this position.
Oh, by the way, ever heard the one about how if you smile, it will trigger the start of happy chemicals in your brain? For many people that's true, to a point, though it doesn't overcome extremes, of course. For people on the spectrum, though, I saw discussion of a study showing that we tend not to be that way. In other words, to a certain extent what she says is true for her. What she doesn't understand is you are a different person.