Taking polygraph tests is a deflection. Your problems are so difficult and so important that you will fall for any charlatan who hints at a solution. 

I mean, what would she say if you eventually passed more tests than you failed? She'd still say you cheated, because you failed *some* of the tests. 

Perhaps you should point out that you did not accuse your SO of sleeping with all the other examiners to get all those "failed" results. 

If your SO is more interested in being right than being in love, then your relationship is in deep trouble and probably cannot be saved. 

If she can admit that her interpretation of the poly results is basically unfair, then you might consider consulting a marriage counselor if you want outside help. Unlike the polygraph, the science behind psychiatric counseling, weak as it is, exists. 

But beware: if you and your SO are not unanimous in wanting the relationship, then even a marriage counselor would be a deflection. 

Note: you'll know it's finally over when she accuses you of sleeping with the marriage counselor to get the results you want.