Frustrated

(Male)I was in relationship for 2yrs with a girl.
She was caring n loving and i had no issue with her at all but the problem is another girl got my attention as she is more beautiful .Even i too loved my gf till within a day i fall for that girl and now im in relation with her.
But previous one is forcing me to be with her. She is emotional blackmailing me i tried my best but i failed in all possible ways to get rid of her.she always cries. She said i cheated her hmm whatever just tell me how to buzz of her from my life.
Why dont she just fit in her mind if i left her, just deal with it rather than cursing me.
Whats wrong if i left her.A person can fall in love with another person anytime irrespective if he already has a partner.
I am so frustrated either i would kill her or myself.