Help needed.... Being blackmailed

As salamualikum

I've made a huge mistake in my life and I don't know what to do.

Astagfirullah; I committed zina. I have no excuses; and I am repenting every day. I am ashamed to say for both, it was 'fooling around; no strings'.

My parents have agreed to an arranged marriage; and I have cut all contact with the girl.

She know tells me she is love with me etc. She has threatened to tell my family - and I know it is the least you think I deserve. I was her first 'partner' which makes things worse...

I don't understand how the girl thinks I would marry her.