@adriw7878,
You already don't trust her. Why the hell are you staying married? Investing in a PI will do nothing but enrich a PI. You aren't going to accept the answer if it turns out she's not cheating, and if she is, then you don't need the private investigator.
And why is she staying married to
you? If I were her and I was being accused of cheating after standing by you for two years of serious medical issues, I'd be looking for the exit sign.