My ex won’t leave me alone

I live in Thika town while my girlfriend stays in Nairobi. Just recently, I found out that she was playing me with this guy who lives in Nairobi. I was so devastated and because of the hurt I went through, I found myself getting involved with a girl who happened to be her best friend.

She was single and was also looking for genuine love. Despite the fact that we sort of broke up after I caught her, she is now becoming a big pain in our relationship, as she won’t let us be happy. She is doing everything she can to interfere with our relationship and she is really stressing us. What can I do about her?