There's this HB8 I've been gaming. When I asked her when she's next free she said "I'm not sure yet" which I took as a flake, however, the next day I kino'd her hard on the way home and she was receptive (didn't move away or anything).

When it came time for us to part ways she lingered around a bit when normally she just goes on her way. I realise now she was probably waiting for a hug/kiss. However, I was cold, tired and just wanted to go home so I wasn't really thinking straight. I just said bye and stroked her arm and she patted me on my arm and we went our separate ways.

After that, when I see her at college she will smile at me, still make eye contact etc (although a lot less frequently than she used to) but she hasn't texted me since or tried to make conversation in person. She's also started favoriting this other boys tweets on twitter quite often and liking a lot of his instagram pictures which may seem like nothing but she never does this to any other boy and never did it to me. She also skipped class today which she never does and I'm pretty sure she probably went to meet him.

Have I messed up? What can I do to salvage this situation?

You've messed up a little, after heavy kino it's best you land a final blow (even if it's just a little hug/peck on the cheeck; this ensures that next time you can open with the same blow you ended on last time and you won't have to start over). She's interested in other guys; you are interested in not only her aswell aren't you? It's no biggy. Just show her that you're interested again and try to put in some kino; WITH a final blow this time. If you like her you should definately go after her. If your game has worked so far just keep on doing what you were doing, but make sure the kino gets heavier, this ensures that she'll understand your intentions and she won't accidentally see you as a friend.
Goodluck!

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Positivity is everything you need