How can I deal with harassment from an ex?

I was in a relationship with a girl for 3 years we were planning for marriage spoke to our respective families..later i got to know that she has been cheating on me with multiple other mens(including few married mens) i wanted to step back from marriage she convinced me that it was all before she met me and requested alot..I forgave her and spoke to my parents about her..later when she got to know that the flat I have is on loan and I don’t have any property under my name without obligation she started saying her family is not agreeing as kundali is not matching I too agreed to it and we decided to get separated..after a couple of months my marriage got fixed with another girl as she got to know about this she started sending her our past private msgs, photos. Please suggest help