How do I stop this woman????

Right 4 years ago I went out with this guy who (to my knowlage) single, we only went out for a fortnight or so but I really grew to trust this guy, he told me that he had 2 kids with his ex and that there was another on the way but she was 9 months pregnant and she'd gotten him arrested when she found out (don't know why!?) Then all the sudden she comes up to me with a bottle telling me that i'm with her boyfriend. He starts kissing her infront of me and then they broke up something like 6 weeks later. But ever since then the girl has been screaming abuse at me in the streets and has hit me. I don't know what to do I'm scared of seeing my Mum, she lives down the same road, and I'm sick of it. I want to stand up to her but I don't know if I have the strength anymore. Calling the police doesn't work. Any advice?