I love my sister's fiance, how do I forget him?

My sister and I are only 2 years apart (I'm 22 and she's 24). We meet him 3 years ago. I was flirting with him in the beginning and we were having a conversation. However, once he saw my sister, he started dating her instead. He chose her and well I can't do nothing but suffer in silence with my fantasy.

However I dream about him sometimes. I'm in love with him that at times I secret go to his workplace building and watch him for a couple minutes. I've been writing poems and romantic stories about him in my journal, imagining what it would've been like being with him, if he had chosen me instead of her. I want to forget him though. How do I forget him?

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Certain days I feel as if my sister took him away from me. We were having a good conversation and it got ruined when she walked in and introduced herself. Ever since then, he laid eyes on her and therefore chose her. I was ignored and just treated a friend or relative.

So I secretly follow him at times, not always.
I love my sister's fiance, how do I forget him?
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