Mono? rapid mono test (by mckesson) started out negative, but doc held it up to the light after a long time and determined it was very faintly positive. i am middle aged, sexually active, only symptom is sore lymph, headache, blurry vision.

Mono doubtful: usually severe weakness, sore throat/pharyngitis. Get actual blood testing. Sounds like you had a (quick) Mono-Spot, performed in the office.

Need better testing: This does not sound like mono and it's unusual to get it at age 42 as most people have at least a mild case by their mid-20's. You probably had Epstein-Barr virus when younger (even in never diagnosed with mono). Get a EBV panel and look for anti-VCA IgM to see if this is really mono. The rapid test has a high rate of false positives. See https://www.cdc.gov/epstein-barr/laboratory-testing.html

Probably negative.: This is likely a negative test. To be certain your doctor can order EB virus antibodies. This is a more specific test for mono.