You cannot truly prove your innocence, because there is no way to prove a negative. Polygraph operators pretend to offer such a solution, but as you've discovered, they're not reliable.
I would suggest that you save your money and not take any further polygraph "tests." Even if you pass, it's unlikely that this will put and end to the trust issues in your relationship.
You (and your significant other) may wish to watch the Penn & Teller Bullshit! episode on lie detectors:
https://antipolygraph.org/blog/2009/02/23/penn-teller-bullshit-lie-detector-episode/
and also this classic CBS 60 Minutes exposé on the polygraph trade:
https://antipolygraph.org/blog/2007/01/30/cbs-60-minutes-expose-on-the-polygraph/
If, after that, you're still set on taking another polygraph, then see our free book, The Lie Behind the Lie Detector (https://antipolygraph.org/pubs.shtml), and carefully read Chapters 3 & 4. It explains how to pass (or beat) a polygraph "test."
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.