My lips are unable to moisturize themselves, hbs, rvs and mp test, what next?

I have a problem which seems like my lips is unable to moisturize itself. It simply just drys up. either crack or get “hard” and then peel. The dried up skin. It seems to be white, worst on my bottom lip, on the middle bottom of my upper lip and also draw a white line on my upper lip when contacted with water.
And inside mouth also forms some little thick saliva.
I also drank water constantly throughout the day.
So i meet the doctor and he ask me to do hbs, rvs and mp test. after i did all, every was negative and the doctor gave me some little drugs, i drank all without success.
but the first time i did the mp test before when i started having this symptom, it was positive, but now negative and the lip problem still remained the same.

This problem has been persisting for the past 4 months.
I’m 23 of age.