I haven't played diana a lot, but she was one of my first mains. I also haven't played much vs trist mid.
With that context, here's my advice:
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Just like pretty much every matchup, diana is going to have a hard time early. Just try to farm up and avoid getting poked. There's a lot of gameplay depth with your q: the best-use for it is being able to cs and poke with it at the same time, but only hitting the low hp minions, so the lane doesn't push (or she'll be able to zone you again).
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Once you hit 6, attack minions twice to ready your passive, then r on her (no q) and auto. She'll likely jump away. Cancel her jump with your e. now it's easy to hit your q. (You should have used shield some time in this sequence). Now walk away as your shield fades. Wait 25 seconds and repeat. Most likely, when you repeat, you'll be able to get a kill, unless she has a massive amount of pots or something.