Welcome to HCM!
Ater going through your query carefully I found few important points: you are 33yrs, 90kg, having slip of right thumb while writing from nine years, and you chew gutkha.
if I had been treating you I would have done a through examination of your hand and thumb, get an X Ray
wrist! hand and thumb (right side ) along with nerve conduction study
to check if there is a nerve weakness involved.
this condition does not look like being caused by gutkha chewing! however it is 100% effective to leave gutkha for ones general well being!!
As it is a long standing problem, and you have not mentioned that there is any other mental stress
troubling you, it will be in your best interest to see a neurologist
/ orthopedician. It is almost not possible to tell the exact diagnosis without examination.
please see the doctor in person.
Hope the reply is helpful for you
Feel free to ask more queries.