Girlfriend emotionally cheated?

Last night I found out my girlfriend had still been close friends with someone she'd sexted and had a crush on. She didn't tell him straight away that we were together and when she did, things devolved and he ended whatever they had (she says just friendship but her guilt suggests stronger feelings than that).

Since then her mental health had been deteriorating till yesterday when we talked about why she'd been disconnected from me and depressed. She'd been longing for a connection and hiding this from me for over a month..

I feel stupid for having been so supportive when she was essentially getting over someone else. I feel betrayed, lied to, and have lost most of my trust. She says she's committed to me now and will do anything to regain my trust in however long that takes.

I just don't know how to move forward from here