This picture describes Isobel's situation. Isobel was so in love with Marco. Even though she realizes in time that Celia and Marco's bond is far greater than theirs was, the time she spent waiting on him broke her heart. He stood her up countless times, and she was in denial for the longest time about it. He left her in the dust while she was still waiting for his return to her.
Friendships change over time. It doesn't mean you have done anything wrong. It doesn't mean your friend has done anything wrong. Stop trying to figure out what you did wrong or blame the other person. Remember the friendship you once had as around memory.