i think my gf is cheating on me what should i do?

dump her. if you think she is she probably is, or at least is thinking about it, otherwise she's behaving like a bitch and doesn't deserve you.

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why would you think that?

Provide the reasons for your suspicions and things will become so much easier. Why bother writing a thread that has 0 details in it? You're being too abstract.

In relantioship the most important thing is trust, if you dont have it, I would dump her, or talk to her.

You should make sure she is cheating before you make any decisions...

If you truly feel in your gut she is cheating, then she is. Period.
I'm not talking the little insecurities all males feel at some point, I'm describing that feeling at your gut level that is undeniable. Save yourself the headache and end it. If you want to get a confession, you usually can. I once (when I felt my girlfriend was cheating LONG ago) used the tactic of telling her I knew (never being specific because I actually didn't know details) what was going on, and if she wanted any chance of continuing the relationship, she should admit it. After a bit, she did, and she broke down crying. I ended it. Just stick to it even when she tries to call your bluff, and keep your poker face. Good luck