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You are feeling the natural depression of grief from the loss of your happy relationship.
If she is not wanting to get back with you then she should not be staying in touch with you. Perhaps she feels that she made a mistake and wants back with you.
The best thing for you to do is to ask her why she contacted you? If she gives you anything positive to go on, then move with it and see if you can't get back together.
If she says that she was just saying hello but has already moved on, then simply tell her that you would like to move on and if she is not interested then it would be best if she stopped contacting you.
Try to make new friends or hang out with old positive ones. Get more exercise, as this always helps to reduce sadness and anger.
Let me recommend a great book for you that has helped many of my clients to move forward with their lives after a breakup:
You will move forward with your life. Stay positive, do positive things, don't self-medicate with alcohol or drugs, and try to watch truly funny movies or read funny books. This really does help.
You will look back at this moment in the not too distant future and it will seem much less significant. Life is like that.
I wish you great success.
Elliott, MAE, LPCC, NCC, CCMHC