失心他梦
[shī xīn tā mèng]
失心 (shī xīn) means 'lost heart,' implying one is so obsessed that they have no mind of their own; 他梦 (tā mèng) means 'his/her dream.' Altogether it conveys the sentiment of being so in love or lost over someone that it almost feels like living in the person's dream, losing oneself.