Firstly I think Anastasia is a beautiful name and it’s all about how you shorten it, so i don’t think you should change your name, you could go by any of the ones below
Ana
Nasa – aNAStAsia
Asia
Sian – aNastaSIA
Saan
Sana

I’m showing you how i came up with those using the capitals and rearranging. there are a few options available to you, think about it. All you have to do is introduce yourself as whichever one you pick and tell your friends that you want to be called that. Be firm, don’t respond unless they call the name you want. It might take a few months, but eventually you’ll be called what you want and your friends will stop making fun. Good luck