How to play AP Melee without getting poked to death?

I need some advice from higher elo melee AP Mid mains

so im a diana main, and when im doing my placements in higher elo, i get poked like crazy, to the point where im zoned from farming even at level 1. If I go for a minion, i lose hp. If I farm under tower, I lose CS. The worst part is, I cant engage till 6. What should I do? Im doing my placements again soon, Ive considered playing someone like Orianna in ranked, but im not as confident as i am with her as I am with Diana.

This is like my only issue, Ive managed to master every part of diana, being a 500k main on 3 diff accounts, this is like the4 only thing holding me back