How can I forget him?

We made good memories and had wonderful conversations and times even though it was a short time. I have never felt like this before. So, I think he lost interest in me. He left me alone. I'm in pain now. And I have mental problems, so if I think a lot about it and make myself sad, I will have attacks and will be hospitalised. How do I stop thinking about him and forget him?
How can I forget him?
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