His decision may or may not have anything to do with drug use. Sometimes men just do this stuff, and it IS as simple as "I don't see a future here / She's not 'the one' / a relationship should be easier than this."
while some women just try like heck to make it work No matter what.
It already wasn't working for you. You were stressed, he wasn't spending time with you. If he was studying, using drugs, partying with friends, or he dropped out of school and was ashamed to tell you and spending his time in the park, does it matter?
I'm sorry you were sand-bagged, (clearly you were). Maybe he felt you were going to break up with him, and he just had to beat you to it.
When I started dating after being widowed, I met a variety of people, not all of them honest or kind. I was looking to a friend for an explanation of one sad incident, and he said, simply, "You don't understand this and you never will, because you would never do this."