He had managed to keep this a secret from me for a whole year and a half
the above indicates that since the entire 18 month span he was getting high, you never got the chance to KNOW him as someone not under the influence. that's some pretty sneaky business, all lies and coverup. yet you say he is your soulmate? how would you really know?
you've taken some really good steps to protect yourself. HE has a long battle ahead of him....kicking that crap to the curb for good. it concerns me, based on experience, that while he SAYS he done for good, he's spending an awful lot of time trying to "win" you back. that's very distracting when one is in the business of recovering from heroin addiction. it's like he has two competing priorities....staying clean AND not "losing" you. there's even a chance that he has the two tied together....
take your time. maybe ask to cut down on contact. don't be his therapist. enjoy your friends, family, hobbies, interests.