>I’m just trying to understand what I did wrong that made him lose interest in me

Simply put, you did nothing wrong.

Things change. People change. Emotions change.

It's not a bad thing, but it is life.

You are both still so young, and who you are at 19 is different to who you were at 16. And who you are at 19 is going to be different to who you are at 22.

If he doesn't know if he wants to be in a relationship, then you can take control of the situation for yourself and end it.

Focus on yourself, and you will eventually find someone who wants to be with you.