Losing a 4v5...

So I just lost a match where the other team had 4 people. No single game has made me lose my confidence as much as this game :[]. I'm mid (afk ended up being bot) and I ended up winning my lane and I was able to roam and help other lanes get kills. Then I don't know what went wrong, but we ended up losing every team fight. I'm a Kassadin main and I ended up with the best kda on the team at the end, but it was only a 9/8/15 so it doesn't really mean much at all. Their Jax ended up with around 25 kills, and I couldn't do anything to shut him down. I main Kassadin. Should I have been focusing on their lux before their Jax? We typically as a team went for the same person, yet Jax would completely obliterate us, with his back up team (Karma, Garen, and Lux) just adding in the extra oomph.

I've carried games far worse. I'm typically the one who wins the game for my team. I just don't understand how I got dominated in a 4v5.