Had mono 21 yrs ago. Now, fatigued, constant headache, etc. Wbc high, luekucytosis. What is test for epstein barr? What happens if not found early?

It is something else. You are done with the mono thing.There is no such thing as chronic mono. Your lab is suggestive of another new illness, but most of these are transient viruses with no available treatments.
EBV titers. Mononucleosis is a rather extreme manifestation of ebv infection. More mild forms can just be sore throat and fever. Once infected by a virus we will often, but not always, develop lifelong immunity demonstrated by high levels of igg antibody against ebv. In the initial, infected phase you would expect to see high levels of igm antibody against ebv.
Not likely mono... What you have now is unlikely related to mono you had 21 yrs ago. Though a monospot test can be done, it has a high false positive rate. Testing for different antibodies (igm and igg) against ebv parts, is more reliable way. However, there is no cure to an ebv infection- only supportive management. Symptom anxiety is likely extended if you don't get a timely diagnosis- not much else. , .