How do you forget someone easily that is still constantly in your life?

so my best friend has a brother and i used to like him before i was her friend, i tried to end this feeling, with time i forgot, but whenever i saw him with other girls it makes me very sad and emotionally shaken. this friend of mine has suffered a lot because her "friends" were only her friends because of her brother and I had to build our friendship with a lot of effort and patience. I got to a point where I can’t take this unsolved passion anymore, how can I end it? note: we used to sleep hugging and being affectionate, but he started dating and it all stopped, so whenever i was almost forgetting him, he was more involved with me