What dose is in your 30 units that you use? 3 months isn't that long, so your withdrawals will likely peak between 2-3 days after your last dose, and you should start feeling better after that. For your habit you should try to stop "cold turkey" as others have said, but if you use medication you should only use it for a few days. I have done a 4 day methadone detox that was relatively painless, but using any longer than that can complicate things by just prolonging the inevitable withdrawals. A 2-4 day suboxone detox is another option, but again, only use it for a few days otherwise it will just complicate things. Part of withdrawals is from using high doses, and another part is from the amount of time on opiates, so a long taper may not be a good idea since it will lead to a longer period in which your body becomes more accustomed to operating with opiates in it, making the transition to having no opiates in your system a little more difficult.
For information about opiate withdrawal and ways to ease the symptoms, check out
this thread.