Alaia I am so sorry for you. I know your hurting but trust me when I say this too shall pass.
One boundary I have FIRMLY established in my life is I no longer allow people in my life that want to hurt me with either words or actions. Life is too short to stick around for the stick in the eye.
Go NO CONTACT TODAY! Sounds like the new girl is throwing alot of salt in your wounds and enjoying it. Just remember what comes around goes around and some day this girl will be in your shoes.
As for you sweetie, you didnt do this. I think the hardest lesson to learn is NOT to take it personally when someone does something like this.
I am treading lightly here but I think (correct me if I am wrong) that even now when you look back you can see the red flags and alot of your pain stems from blaming yourself for not seeing the flags then. Maybe look at it this way, atleast NOW you see the flags and its kind of like a rehearsal for the rest of your life. You will be able to spot people who mean you harm sooner before you get emotionally attached...
Someone once said that life is about lessons. When we learn a lesson we move on and continue building on that one lesson. Some lessons are harder to learn and we keep repeating the lesson until we get it. Take this lesson and build from it. Take care of yourself. Love yourself. Be true to what you need in your life.
Today is a new day. Start fresh. Look at the past as just that the past. We all make mistakes. Dont be too hard on yourself.