Mom isn't supportive

So my ex and I are talking about getting back together and I thought I'd tell my mom. She instantly shot it down. Said I wasn't allowed to get with him again. Called him every name in the book. Asked if I was stupid and now isn't speaking to me. I'm 18 almost 19 I don't understand why she thinks she has a choice in who I'm with. It's not like I'm moving him in her house. My mom is my best friend so I thought she would be supportive of the idea.