I suggest that you contact a sex therapist.

This is their job: to help sex get better, or get back to where it was. They are familiar with low sexual desire; they are familiar with people dealing with affairs.

You suggested that the situation might not be important enough to justify seeing a therapist. In my perspective, an inability to enjoy sex for many years is an important concern. I think that you deserve better, and I think that a sex therapist (or a regular t who is familiar with sex therapy) can help.