Okay, so my mum's birthday party was not last Saturday but the one before that. My best friends birthday was the same day and my mum had told me to the choose which one to go to. I decided to go to my friends because all other best friends would be there and there would be no way to get there if I went to my mum's for a while before. But a few days before the party my mum told my nanny that I was told to go for a least an hour and as much as I wanted to there was no way I could go to both. I could have just stayed at my mums but it was like an all-nighter thing and none of my cousins my age would be there and I would be desperatly bored.
So.. my mum and I got into this not talking to each other thing and stayed at my dads for the last week and a half. There wasn't much contact apart from a few calls that didn't last long.
I was home last Saturday getting some stuff and leaving of my brother to her house. She didn't seem annoyed and I apologised but stayed at my dads house that night. I came to her house today after school and I'm here now.
It's been fine-ish but she keeps making me feel guilty and I don't know what to do. I was telling her about me not sure if I would be able to go on holiday with my dad and brother (explained on my other latest question) and she said 'well, if you came to my birthday I would have thought about bringing you and me away but the way you acted was unforgivable' and I was just in shock feeling like I was going to cry. She stresses me out too much.

Well my question is, what can I do.. To me, talking with her isn't really an answer as I find it hard to talk about things I'm upset about with her. I feel horrible and I just don't know what to do.. please help.

She likely feels that you like your friends more than her, and is feeling a little neglected. If I were you, I'd bake her an "I'm Sorry, Happy Birthday" cake and offer her a thought out apology.

I would but I'm a hopeless cook. I'm quite the stubborn person and right at the moment I feel like why did I bother apaulogising before because it hasn't done anything. My mum and I can be really close at one minute then be annoyed at each other the next. Its an upseting thing and quite hard to deal with at times.

"I've been thinking about the choice I made on missing your birthday. I know you understand why I went to the other party, but it really didn't occur to me how hurtful it would be to you. I realize that now. I am so sorry I hurt you, I truly hope you'll forgive me. It was never my intention to do hurt you this way. I hope you'll forgive me."

Apologizing when you don't feel like you did anything wrong is a growth experience. It is a necessary skill you will have to develop to live in the house with a person, especially later when you are living with a spouse whom you wish to have sex with later that day/week.

Apologizing to a loved one for a perceived offense is a necessary evil. You need to be able to do it, to sell it sincerely, and not feel like you're "losing" when you do it. It's not a loss. Being able to set your pride aside to comfort/console the feelings of a loved one IS SOMETHING you will learn to do, sooner or later.

So, why not sooner? Mark Twain said, "Always do right. That will gratify some of the people, and astonish the rest."

You still go to go to the party you wanted to go to, right? Now you can get peace back with your mum by simply apologizing a couple of times and meaning what you say. Apologize to your mum for making a choice that hurt her feelings. Mean it.