Ever had a friendship go sour? If so why?

I've had a few. One I was on a course, which I really struggled with, mainly because I'm not very smart but my relationship was stressing me out, my mother kept running up our joint phone bill in my name, and asking for money for other stuff, the girls on the course we're clique. We got talking and she wasn't very confident, non the less was a nice person blunt at times but I feel it was very defensive.
Soon after she started manipulating the friendship between me and another girl in our "group". Subtly insulting me about my acne scarred skin showing those posts about makeup cheating men, on instagram saying women like that don't deserve someone good, coz deceiving him. Any suggestion I had for an activity or conversation topic was childish, stupid annoying. When I'd sit at a table with her she'd ignore me. Started running off and would leave without saying despite us walking Home together everyday. When u confronted her about these things she said it as all in my head, that I was just being sensitive.
I stopped talking to her after I gave her a second chance and she disrespected me again at the dinner table in front of others.