Alhamdulillah, it is very encouraging to note that you have given up this evil action. May Allah Ta?ala keep you steadfast, Aameen.
You should use your discretion to determine how many such fasts were broken from the time you became Baaligh (mature). You should act according to whatever the dominant part of your mind tells you. Thereafter, make Qadhaa only of the number of fasts you had broken.
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai