Just come clean and tell her you looked at her phone and you know what she's up to. At this point it doesn't matter that you were snooping in her personal business. Time to sack up, my friend. She's cheating on you. It doesn't matter if it's with a woman, it's wrong because she is having sex with a person that isn't YOU.
Honestly, I hate cheaters so much. I say just kick her ass out, even if she denies it the whole way. She obviously has no respect for you and doesn't think you're smart enough to figure it out. You need to get angry about this (not violent angry....just offended angry), not sad and sick. You don't sound like you're standing up to her at all, she's giving you lame denials and you're letting her off the hook. Stop doing that. Call her out on her shit and take a stand.
Live as if you were to die tomorrow. Learn as if you were to live forever. - Mohandas Gandhi