definitely DO NOT do this. if she isn't cheating, let it go. if she is, flat out break up with her no questions answered just 'it's over.' if she's any type of girl she's probably going to tell you she had been cheating on you. if she does, well, i guess you could lie and say you were too, or just that she's boring and you don't give a shit. if she doesn't say anything about cheating, well, don't say anything AT ALL. even if she has absolutely no feelings for you it will drive her mad.
paranoia is common in insecure people. you will look for a clue that gives you the answer to what you really think (i.e. she's cheating). when your mind is settled you'll probably still be paranoid, but you'll have this to fall back on and remember that you're wrong about what you think she's doing.
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