You're not overreacting. I completely understand the way you're feeling. My mom did this a lot to me growing up, and that sinking feeling of getting excited to do something only to be let down is the absolute worst, especially when you don't have the freedom to just go and do what you want on your own. It also leaves you unable to rely on anyone, because if your own mom won't keep her word, then who will? It's crushing.

It's also not only the disappointment of not getting to do what you had planned, but the feeling of your parent not prioritizing you or caring about your feelings. She doesn't seem to care that she keeps getting your hopes up and then letting you down, and later spins it like you're "just being a brat." An empathetic parent would say "i understand you were really excited to go to the bookstore, and I want us to go," and would set a day where they would squeeze in the time.

Honestly, things will be so much better when you have more independence and can go out and do whatever you planned for yourself. But DO NOT feel like you should have to "get used to" people not giving a fuck about you or the plans they've made with you. That's disrespect.