u/Intrepid_Entrance830

Am I The Asshole? I'm a teenager, and my mom flew to the Seychelles to celebrate my birthday with my stepfather. She's arriving tomorrow, and they locked the room so no one could come in. Grandma stayed with me for the few days they were gone. She knew how to get in. I bought balloons today and wanted to make a surprise party so they'd walk into the room, and I'd be there, decorated. I told my aunt, uncle, and grandma about it. Grandma had gone to language school to learn Spanish, and she wouldn't be back late. I decided to take the key from the key holder under the TV where all the keys are and decorate the room while Grandma was in Spanish, and I told her about it. My parents were at the airport at the time. She said she'd tell them right away that I'd opened the room with the key they'd put in the key holder. I started calling her because it was a surprise. I yelled at her in my anger, saying she had no right to ruin the surprise. My grandmother often annoys me with all sorts of things, and she doesn't realize it and doesn't understand why I'm angry, even though she acts like a baby at these moments. I wrote her straight out of the blue that I don't want to talk to her anymore. Incidentally, her joke played a nasty role: "Is it the mother's fault that the child is a bit stupid?" This really offended me, because you least expect to hear that from a loved one. It's okay to hear from friends that I'm an idiot and so on, but not from my grandmother that I'm stupid. I wrote to my stepfather about how I behaved inappropriately, as I really did enter someone else's room without permission, and wrote the following message: "Hello, I am really sorry I did something wrong. I wanted to decorate your room and make a surprise for Anna as it was her bday. I bought over 80 balloons and wanted to start now. I wanted to go and start and took the key from your room under the TV. I understood that it was wrong to do it without your permission. I feel bad about it. I started to decorate your room but now I stopped and I closed your room as it was not right to open it without Olga. I hope that you will forgive me and don't say to Anna about decorations. I am really sorry for this such a stupid mistake. Olga will return back around 20:30 so I will wait for her to continue. I really really fell bad about it. I called you to say about it but I think so that you are on your flight. Sorry, see you in Barcelona!
I feel embarrassed I am sorry
It was unacceptable, sorry." I realized the mistake and really should have waited for my grandmother and not yelled at her. What do you recommend doing?