Even if she was sleeping with someone else, telling her that you pretty much stalked her account and found a guy in the comment section is gonna make you look like the most DESPERATE guy in the world. You're the man. You're nearly 50. Don't do the teenager stunts, please.

29yo with libido problem sounds like a myth, but it happens. It may be it. If you don't trust her though you should end this as soon as possible because what's the point of going on? Trust must be there.