Lots of questions, most of which are personal decisions rather than black & white issues one can answer for sure. But that's life, you make a decision, hope it works out ok. If it does then great, if it doesn't then it's a learning experience for you to become a wiser person.
First off why should you feel any guilt ? You started a relationship with someone whom you understood to be free. Anyway at the age you appear to be I'm not even sure the concept of 'free' is that applicable. When young you are expected to have as many relationships as possible anyway to realise what you really value in another, and what not.
Anyway seems to me you have acted honourably enough.
I'm unsurprised you feel insecure. She gave you the impression she was free of emotional ties from past relationships, yet she seems not to have finalised anything with her ex; and was given enough doubts to have gone back to him for a while. Maybe she tried to move into a new relationship a little to quickly. she needs to sort out for sure what she really wants. It isn't fair to expect to have others dangling on in hope while she works it out.
Whether you can draw a line on this and start afresh, or even whether you think it has reached that stage, is up to you. In your shoes I think I'd at least let it cool for a while, while the dust settled and it became clear what both of you want. But since you say everyone knows she is with him and he has all her pictures you imply she has already made her decision.
It may be best to just leave it all behind. Plenty more fish in the sea even if, at the time, that seems totally wrong. Trust me, time tends to heal, as does a new love interest.