I'm sorry you are feeling so sad. I think before you know it she will be back and you will be together. Maybe you just need to get used to her having others in her life. Sometimes that can be hard especially when you have low self esteem. Have you ever tried positive affirmations. You can write out all your good qualities and affirm yourself that way. Please realize you mean a lot to her too. I hope things will start to go better for you.