Hi. This is a comfort thing which is akin to an addiction. You need to break the habit, as you would with any addiction. Trying gum as sandman2 suggests might help, as might using something else you can do the same action with, for example an iced lolly or one of those hard candy things you can suck on.
If you have always done this it is likely to be a lengthy process to wean yourself off, so don't expect miracles. Give yourself time and keep trying.
Good luck.
Why? probably stress. Try gum. experiment with different types - just be aware of people around you.