A lot of people – and it sounds from your description like your mom is one of them – don’t know how to express disappointment, deal with frustration or criticize except in ways that are hurtful, cruel and unhelpful. Sometimes it’s just because that’s how they were raised: they absorbed the terrible lessons that their own parents taught them, and never moved on from that point. Sometimes it’s a problem with language and culture; in some cultures it is not considered so damaging to children to upbraid them very strongly, as your mom is. Sometimes, let’s face it, it’s because they’re awful people. I can’t judge your mom.
So without knowing more about your mom and her own upbringing – and believe me, I’m not asking for details about your mom and your family life! – it’s not possible for me to say “why she is acting this way”. But really, even if we know that, it doesn’t get us any nearer to a solution, does it?
I’d recommend – as I often recommend – changing the question. Ask instead, “How can I deal with my mom’s hurtful comments?” or “What can I do to calm my mom so she won’t say such mean things?” or even (though I do not recommend trying to answer this, at least it would gain positive suggestions to change your own behavior, which is within your scope of control) “How can I be the perfect daughter?”
And you can probably answer all of those questions yourself, or ask any one of them and get some very helpful responses.