Is my girlfriend cheating on me?

I think my gf of 8years is cheating beacause she is always on a chat site similar to msm and when I go over to her she tilts the phone away from me, or if I want to sit next to her sit comes up with the excuse the she wants to lie down, when iv checked the history on her phone iv found that all her friends are guys now iv questioned her about it she flips out at me it’s getting to the point now that I don’t know if I want to be with her, but we have a 18mnth old son and I know that if I left her she would make it so difficult for me to see him I just wished she would tell me the truth coz if she is or planning to then iv waisted all my teenage life and most of my twenties….. Please could someone give me a bit of advice..

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