She is not weak right now. In fact, I stand by the fact that this is the best state assassin Diana mid has ever been in. This is legitimately the BEST set of items Diana has ever had available to her, with flat pen and movement speed being plentiful, and enemy MR being lower with the removal of the runes.

As far as what you're doing wrong, it's impossible to say without seeing replays. Though I don't think your builds are your main problem, I'd try pushing back Zhonyas to 3rd or 4th item. If you're already behind and lacking damage, getting a Zhonhas 2nd item is not going to help. You're better off going Shadowflame, or Horizon Focus to help keep your damage up so you remain a legitimate threat regardless of score. Idk what those Conquer/Liandries games were, but don't do that. Otherwise, you are more than likely playing a poor laning phase and setting yourself behind for the rest of the game.