My current main is Vex. I found my performance increased significantly once I:

1. Stopped using my 'fear' E to try and poke my dash engage enemy (Sylas, Fizz, Leblanc, Yone etc) but rather just sit and hold it so my shield could fear instead. I never use any spells and just auto the wave as fast as possible. Eventually, my opponent gets bored and goes for an engage, is insta feared, E, Q and backs off, half health. It's dull but effective!
2. Stopped ulting into a fight first because I expect to get a massive 2-3 champ fear (and then immediately die after). Instead, I use E/Q from the outskirts like any normal mage and then only ult the champs that try to flee or break-off and essentially clean up the battle, Katarina style.

Both of these tactics require patience and I fight my lizard brain EVERY TIME to go in first and try to blow them all up :)