I'm not sure "where I went wrong" but my two high schoolers are not as interested in eating veggies or sometimes even fruit compared to their older and younger brothers. I make them eat fruits and veggies anyway - falling back on my greatest tricks to be sure they don't get scurvy. ;)
What are my greatest tricks? Here's one of them, and the simplicity of it may surprise you:
Cut the fruit or veggie and put it on a plate. The end.
Why shoving a plate of prepared fruit under their noses works compared to saying, "Please go eat an apple" I'll never know. But two days ago I sliced a bunch of strawberries, slid it across the counter to Malachi and before either of us realized it, he'd polished off 1/2 pound of berries.
Score!
Now, I'm not into coddling or spoiling my kids by doing their work for them, but I do want them to eat the good stuff. So if I need to take a little time to make their fruits and veggies look more appealing so they'll actually eat them, I will do it. Their health matters to me.
(And for the record, if I'm going to do something extra for them - like cut them some strawberries - I usually send them off to do something for me in the meantime, like put my laundry in the dryer or chase a baby around the living room.)