I knew I could never give you what you wanted.  All I could stare at was the photos you left of us two. Nothing could change the way you made me feel but one day you told me, "I found someone better." And hearing these words shattered my heart. Even though I told you I moved on I can't help but feel the void you left in my heart as I gaze upon you smiling happily with him by your side. I hope you live a happy life Bokuto....

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To summarize Oikawa was with Bokuto until they broke up! The person with Bokuto Oikawa was referring to was Akaashi.
I did this in 15 minutes as a challenge and sent it to my friend except I removed the last part. You don't know how much I regret this oakfowvjqpgknwofod