I can't deal damage with this champion and I don't understand why

There have been games that I reached 2k+ AP in Arena or Aram Mayhem. I dive into the enemy with E W and I end up dealing more damage to myself than the enemy carry. Somehow with that much AP, fully charged E still doesn't take 20% HP from an ADC. I am checking the ability tooltip and it says it's supposed to deal about 2k damage. Where does that damage go? I am so confused with this champion.