okay - I know how it hurts to read that junk when you snoop. BUT - there may actually be a (tiny) silver lining to this cloud. The silver lining is that she didn't want you to know those were her texts.
If she were 100% gone, she'd be more likely to express anger at you and say it was none of your business who she talked to. Instead, she seems to want to cover her tracks - most likely to keep her options open with you.
Now - what to do next? I would: A) Avoid talking about the cellphone again. Steer away from that conversation, or if she insists on bringing it up, just say you really don't care to talk about it right now.
B) Go a little bit dark. Not in a mean or pouting or punitive way; just enough to let her worry a little about losing you. Don't answer your phone much. Be "busy" more often. If she ropes you in to help S12 with his homework again, go to a library or coffee house to work with him. If she asks where you've been, just make up something you have been "busy" with.
