Did I do anything wrong here or is this just something that's gonna happen?
Fold preflop and it won't happen. This hand really shows why playing OOP sucks and it's because you built a pot OOP with a weak hand and built it big enough to pay off his flush when he hits. Doing some math it seems that he called $7 to win $29 given that he had position and you gave him implied odds so his turn call isn't even a mistake here.

If you didn't think he had a flush draw, what did you think he had? Did you really expect him to call with a worse hand on the river after calling a big bet on the turn when the flush hits the river?

Edit: My suggestion for the river is check/calling or check/folding depending on what you know about him.