Is it possible to become resistant to valtrex (valacyclovir) and it stops working as well as before?

Yes, but rare: It's possible but very unusual. It tends to happen only in immunosuppressed patients who have taken intermittent Acyclovir or valacyclovir for many years. If you develop a herpes lesion while taking valtrex, the lesion can be cultured and sent to the lab for sensitivity testing, which will determine whether the herpes is sensitive or resistant.