Can you really put the past behind you?

Im wondering, if you had a serious drug backround would it be a good idea to tell someone your in a relationship with/? if so how would you go about this? i was addicted to heroin for almost 2 years and now im clean and have been for almost 6 monthes and i have a boyfriend and weve been dating for about a year ( he never knew) and i want him to know but if its not a good idea to tell him, well then i wont. i mean i never got arrested ( had some close calls thiough!) but i dont want his opinion of me to change...