My friends made party and left me out

Solo, I'm kinda guilty for this one but I'mma explain so I can fully understand my mistakes and how to not repeat them (my first language isn't English, sorry if it's a little hard to understand)

So, my friend made a party yesterday. You know , normal Halloween party. She planned this 2 weeks ago, but I already had plans. Also, she didn't tell me what she wanted, she just said "are you free on Halloween" over and over again. I told her I had plans with my friends but I wasn't sure. She told that as long as I told her some days ahead the party that I was free, I could go.

On Saturday, I told her that I was actually free and could go. (She literally told me "oh then you probably can come, I have to check though"). Saturday, doesn't says anything (I told her at literally 10 am). Sunday, same. At Monday, I asked and she said "well I don't know, give me time" and I was like wtf, I just want to know if I can go or to make other plans.

On Tuesday she didn't say anything. Not that I could go, or that couldn't. I felt obviously let down. They've made photos and keep uploading them and I just feel bad.

Sooooo, help and tell how can I improve so this doesn't happen again or idk