Until you are able to make the connection in your brain that drugs/heroin make you feel BAD, it's hard to avoid missing your DOC. When you think of using now, do you feel a flood of good things? Time for a change and you can start working on this right now.
Think back and really remember the worst experiences you had while using, both on a physical (feeling sick) and emotional (feeling anxious or afraid) level. Just absolutely immerse yourself in these negative memories...try to feel sick, frightened, scared. This is your new line of defense for cravings. Every time you feel the need to use again, call up these bad thoughts. If you do it consistently, really work at it, you'll be surprised at how much easier it gets to say no.
This method helped me more than anything else. Now when I see, smell or think about wine (that's all I drank...lots of it) I get overwhelmed with nausea, headache and anxiety. I can almost smell vomit (gross, I know, but it's true). I know that there is a scientific name for this (aversion training?), but it's something I did on my own, and it sure helped me stop!