How Do I Decrease My Tolerance?

For context: I am an 18 year old student, 1.93m tall and only weigh 70kg. I have been heavily drinking daily since I have been 16 but this has not interrupted my daily tasks with University etc… These days I will kill a 1L Smirnoff across a 1 night period and not even be blacked out just (decently) drunk. I miss being blacked out so now I just take Ibuprofen before drinking. Any tips?