How to stop thinking about my brother-in-law?

My sister, placing her trust in me, requested that I look after my brother-in-law while she was away for work. As a result of spending extensive time together, he managed to seduce me, leading to an instance where an unexpected embrace led us to overstep boundaries. Despite understanding its impropriety, I succumbed to his advances, deepening our mutual feelings.

It was also the time when my sister and he frequently engaged in disputes, amidst which he expressed a wish that I were his spouse, professing his feelings for me yet acknowledging the impossibility of leaving my sister due to their marriage.

Despite being aware that our relationship has no future, I find myself unable to move on, haunted not only by our moment of intimacy but also by his continued subtle acts of affection, which has convinced me of his love, a sentiment that ultimately brings pain.

With my sister and her husband now reconciled, their happiness fills me with sadness and stress, particularly when they share moments in their room. My inability to relocate and my reluctance to distance myself from them, fearing it would leave me feeling lost and sorrowful, have trapped me in a dilemma.

Confiding in family members is not an option, as revealing this secret could jeopardize my sister’s marriage. Longing for a relationship with my brother-in-law is not practical either, so I am left questioning how to erase him from my thoughts and find happiness once again.

What should I do?