How to play against assassins as a mage in midlane
As of recently I have gotten into league and have primarily been playing Midlane as a mage. Currently I am only at bronze 1 and generally have no issues with fighting mages and assassin alike. Now out of nowhere I've begun encountering assassins who put an insane amount of pressure on me and constantly out CS me our kill me 5-6 times. Its beginning to frustrate me to no end.
I know I am not great considering I have only been playing for 3 months but I am extremely lost on what to do. If I stay under tower I end up losing a good amount of CS as I either don't have enough mana or my cool down limits me from taking minions. If I try to be aggressive and keep my CS up I usually end up killed.
For more info I usually play Veigar or Xerath midlane. My worst matchups so far have been Fiz and Irelia.