Should i stay with my finace?

So today i find out that my fiance has been taking narcotics again which he was good for almost a year. But now i get this news he finally told me its been about 1 month and a half since he has been using again. WE are supposed to get married in october and we have 2 kids. So many things has been going on then he drops this on me and i am so confussed on what i should do. Should i leave him or stay and help him. It's just that i have watched so many of my family members get torn apart from drug addiction and i know what it can do to the addict and the the family. I am so scared and confused. It has ent gottin to bad. But it could and i dont want it to get to worst. Any Advice?