I originally thought Valtrex would have less side effects than Famvir, but have since read that Famvir is often a more gentle choice?
My ME/CFS specialist tries Famvir first in most cases because it appears to be better tolerated. However, she also said that Valtrex works somewhat better for EBV so if you've taken it before and tolerated it or if you don't improve on Famvir, she uses Valtrex.
If Valtrex is giving you trouble, I'd suggest trying Famvir instead. A lot of PWME have used it successfully for EBV.
You might also consider trying a different Valtrex generic. People seem to be reporting more trouble with the brands that are blue... I don't know why. Maybe they are made by the same, slightly different, formula, or maybe it's the dye. Who knows? Another ME/CFS specialist I saw, who has been treating ME with antivirals for a very long time, said that the Mylan and one other (I forget which, maybe TEVA; it wasn't blue) seem to work better for his patients than other generic brands. As far as I know, there's no research behind these opinions, just clinical experience.