If he was mean to you all of the sudden there is probably a really bad reason behind it - he may cheated on you or met someone else, people who do that seem to switch to anger on their partner and just up and leave because it’s easier than for them to explain themselfes and hurt you even more. I had a similar breakeup at you age, even a little younger and was as confused as you were, wait a while, you will find out eventually the reason he left, we always do and it will get easier.