Am I a Horrible Person?

One, your mom doesn’t have a right to get mad at you for being upset. My dad does that too, and it’s extremely invalidating. She has no right to call you insensitive and especially not a cunt, just because you felt she was being mean. She chose to react you you being upset by getting mad, that is not you blaming her at ALL. That whole second paragraph is her being emotionally manipulative. You are aloud to be upset, spilling food is frustrating, you no longer have the food you were going to eat and you have to clean it up. It can feel overwhelming. It isnt you who make people feel horrible, you are aloud to react to things how you want. My mom used to be like that for me too, every time I stood up for myself or tried to explain myself, it was always my fault and I’m making HER life worse, when its never about her in the first place. Your is treating you poorly and almost straight up gaslighting you. I know me telling you all this won’t immediately change how you feel, I know you’re still going to doubt that it’s your fault. I highly recommend starting to keep track of when she does this. Recognizing that there is a pattern was a huge help for me personally. If you need to talk I’m here. If you can talk to someone you trust as well who doesn’t known your mom that well is also a good idea, because they wont have many preconceived ideas about her.

Look, I don't think you're being too sensitive. If what your mom did/said while she was irritated with you while you were visibly distressed hurt you, you absolutely should be able to discuss that with her to find a more productive way of interacting with each other. Obviously for your own sake you should work on coping mechanisms for being overwhelmed and such and you shouldn't expect others to fix your problems, if that was genuinely your intentions, but your feelings surrounding the situation and how your mother reacted are still valid emotions and you should still be able to communicate that to figure out a comprised to avoid these falling outs in the future. Including HER acknowledging and changing the behaviours that hurt you.

For you currently have a therapist

This seems very sketchy. (The things your mom is saying.) When someone convinces another person they are being 'too sensitive', way more times than not they are using it as a manipulation tactic (intentionally or not). There are a lot of other red flags in your mother's behavior and I'd encourage you to look up emotional abuse/neglect just to see if anything lines up with your mother's behavior. I'm not trying to convince you of anything and I don't know more about you and your mother's relationship than this post, but it seems like you need a direction to take. I hope things get better for you! No matter what I don't think you deserve to be called what she called you anyway. Remember your emotions are important and valid no matter what anyone says. Anxiety sucks. ❤️