Hi,
I'm trying to find a yaoi manga where the seme is in high school and he needs to 'suck' life force/essence out of other people by kissing and getting intimate etc. Because of this seme has many different partners. He finds uke tastes the best and they have sex quite often. I think they had a fight as uke felt that seme didn't love him, he was only using him for the life essence thing. Uke left seme alone, after a while seme collapsed from exhaustion and uke discovered that seme didn't get life force from anyone else. They reconciled in the end.

Is it Katsuai Monster?

That's the one, thank you so much!