Firstly I congratulate you for this thought. 12 years is a long time and it takes lot of will to overcome it. I appreciate that you are willing to quit. Gradually try reducing the habit.... Say if you chew four times a day.... Make it 3 for a week then 2 for a month and then one and finally make it one in 2-3 days...then one in 10 days.... Slowly try reducing. Replace tobacco with nicotine gums. Go for regular counselling and checkup.... That will help you

U need to quit tobacco, as it leads to oral submucous fibrosis, which is a precancerous condition.  U can try nicotine chewing gums, or approach rehabilitation centres, for quitting the habbit .