This girl likes my boyfriend...what should I do?

[Alice]

This girl is a total b**ch to me..she always flirts with him infront of me and it pisses me off so much!
All she does is bitch about me and call me a "Lesbian" because I dye my hair and have a septum piercing. She's his ex and she said she hated him but yesterday she posted on Facebook: awesome Miles listens to MCR! He just became more attractive!

It pissed me off so much I cut...cutting isn't a big deal for me I've been doing it since I was five...
ANYWAY!

Me and him were sitting on the couch yesterday watching Sherlock and then he got a text..he said he wasn't going to look at it but then..another..and another...another...and another..It was her..I took his phone and turned it off for the rest of the show then afterwards turned it back on and looked at the texts

Scarlet: Hey baby.
Baby?
Why wont you text back?
Baby come on..
Baby!?
Are you hanging out with that slut again?!

he took his phone back and texted her: Scarlet stop being mean to her. She's my girlfriend so stop calling me "baby". Never text me again. I never want to see you near her or talking to her so f**k off!

She somehow got my number and started texting me to kill myself.

Me and him have been together for two years with only one brake up..people say we're perfect together..I just need some help with this..I really dont want to brake up with him..!
I'm 18. She's 16 and he's 19