Your are a diabetic since 2 years and your are no metformin
since 6 months.
Metformin is known to cause Vitamin B12 deficiency
which may cause glossitis
and cause dry mouth
But it is to early for this to happen.
Regularly take Vitamin B12 supplements.
Monitor and control your blood sugar.
Polydipsia(excessive thirst) may also cause dry mouth.
For nasal dry congestion consult an ENT