How to forget a person that you truly love?He had somebody else and it really hurts me!pls help me!?

I had a friend named John.At first I only consider him my friend but later on i learn to love him.My friends told me that he loves me too because he helps me whenever I need his help. But i had my doubts. It feels that John helps me because he considers me to be his younger sister. As time goes by,we became close with his sister. His sister would invite me to go with them. Then it made my feelings stronger for John. I don't know what to do since i'm really in love with him. As months ped by, i just know that he had a girlfriend. It really hurts me! I just pretend to my friends that i'm alright but deep inside it really hurts. I also know from my friend that John's girlfriend had the same personality and perception with me. In other words, we had many similarities. Ever since that incidence I would not communicate or talk with John because it really hurts. And i've decided to forget him but its difficult since his sister would invite me to go hang out with them!Pls help me!