How do i stop my big dog from attacking other dogs?

hi im only 15 and i take my bordercollie x kelpie chilly(u can see her as my pic)for walks everyday. she never used to attack small dogs and rarely got in fights with big ones either but a few weeks ago i was out side my friends with chilly and my friends 2 jack russels when the people across the streets dogs got out a lab and a blue cattle dog and attacked my friends smallest jack russel which only survived because she had a jacket on which got ripped right off her then went for her other dog chilly was trying to get to them 2 help the to dogs but my friends dad who was hittin the dogs with a broom(and broke the brrom on the cattle dogs head lol) told he 2 keep her away but she broke our very think lead and attacked the dogs but there was to of them and shes an old dog nearly 9 and they were bother on top of her and i was hittin them to try and get them off but they wouldnt and the whole time the owners were watchin and laughing from across the street they finally got off my dog because another neibour came and dragged them off my dog she had half her ear ripped off and bittes all over her and i took her home but ever since every time we see a dog she doesnt no she attacks it big little puppy she doesnt care anymore at first she would hide behind me but now she goes straight for them i dont no what to do and shes a big dog she could kill a small dog but because of her breed she needs to be walked now i dont let her off the lead but i want 2 be able 2 because she needs to run shes becoming overweight and has to much energy i dont no what 2 do though i dont want her to hurt another dog its so hard:( plz help me