If you realize you’re no longer romantically interested in someone, you should break up with them — be that a year or more in, or, as seems to be your case, one day. You can’t undo what you did in your moment of panic and confusion, but it is actually a kindness in dating to end things politely but directly.
It’s hard to tell from what’s written, but have you even spent any time together this summer or has your friendship and now relationship been completely over text? I’m assuming you’re both very young, so yes, there may be some hurt feelings, but I don’t think it will be as bad as you seem to be expecting. And, assuming you learn something from this experience, you’re not an asshole!