I totally disagree with the wisdom of everyone so far. She still has an effect on you because the person you saw is the one in your memory, not the actual human that you saw. You're not 16 any more, but it is your 16-year-old self that is being tormented. Call her, go for coffee, find out what she's been up to in the last three years. Nothing removes the burning of an old flame more than seeing them as the human being they actually are. Also, thank her for the great summer. It was a stepping stone in moving you towards the person you want to be. So, I saw my first real high school romance today for the first time in about a year.
I assume by "saw" you mean accidentally, across the street, or something like that.but I never ask her to be my girlfriend.
Whatever that means. You were 16. She was older. Maybe you were afraid she'd say no. Maybe any number of things. Methinks you have some unfinished business. You don't have to finish it, but you might want to recognize what it is you want to say to her or to yourself about all that.
As a man whose primary hobby seems to be collecting ex girlfriends, I think this is good advice (although, I would recommend going out for a cocktail instead of coffee, because it makes an easier transition just in case things go well). Probably what will happen is you'll fall in love once again, but then after date two or three, you'll realize she's not at all what you remember, and you'll want out. Perfect cure.Call her, go for coffee, find out what she's been up to in the last three years. Nothing removes the burning of an old flame more than seeing them as the human being they actually are.