To be honest, I'd probably not tell her anything at all. Do you think it'd make you feel better knowing what she was doing over summer? It sounds like she wants the best of both worlds, she wants to be single and do whatever, but then when you're both back at school she wants you around then... at her convenience, not yours. Whatever relationship you guys have seems to be all about her, not about you.

You're right that finding someone is "staggeringly difficult", but remember that it's not the be-all and end-all of life, and surely you'd prefer to wait a while and find someone that truly likes you rather than someone that is quite happy to play mind-games with you and make you feel bad?

It will almost definitely take you a while to get over it, and don't feel bad about that. But before you have anything further to do with this girl, just look out for yourself ok? Don't let her hurt you, you deserve way better.