A little background: I feel like a combination of both a shitty person and a naive one for getting into this situation. Granted, I've never had a healthy relationship (i.e. mutual attraction) and have always dated whoever would have me. The last girl I tried to date strung me along for months (she later admitted she wasn't into me from the beginning). Before that, I dated a girl who threatened to harm herself if I broke it off with her. And before that, I never dated anyone (i.e. first relationship at 26). In short, I don't know how dating is supposed to work.

Present: I live in a major city and I signed up for a few dating apps. While I went on dates every few weeks, I didn't really click with anyone. Then about a month ago, I found two really great girls. On the advice of friends and because I couldn't decide on a better fit, I kept dating both. The problem is I like one, but the other one thinks we're already really serious. She thinks this because she doesn't do casual sex (I didn't really know this), but we drunkenly hooked up on a third date.

The one I want to break it off with clearly likes me a ton. She texts quickly despite my long pauses, is always asking when I'm free, texts when she thinks of me, and drunkenly said she "worries I might cheat on her" (I didn't even think we were serious!). Anyway, we're both kind of quirky people, so I can see how she feels we have something very special (she says how she doesn't know anyone like me). So I'm afraid she's going to be devastated and has no indication anything is wrong...

Today she started asking if I'm free tomorrow or Thursday. I don't know what to say at this point. We've gone out since mid-August and hooked up a few times. I want to be respectful, I didn't intentionally string her along, I feel awful, and I just don't even know how to handling it. Text, call, in person? I know she's just going to sob and beg me to reconsider.... please help

tl;dr I've never had to reject a girl who gets along great with me and I'm at a loss

EDIT: So far I've learned that everyone has very different opinions on breaking up. It feels like there's no objective consensus on how to do it properly (in a way that there is often consensus on other topics in this sub). Some people say be fully honest, some say give no details, some say they want a text, others in person. Maybe /r/breakup_advice needs to be more of a thing, because I don't think people are generally handling this well.