Hi One and welcome! I can really relate to your story because I have lifelong pain issues, Fibromyalgia, migraines, etc. And I too wound up with a huge oxy addiction taking about double the amount you mentioned, snorting and injecting it. So I know where you are coming from.
Like the others have said, we can't give medical advice and seeing a doctor or an addiction specialist would be your best bet for tapering. I chose to use suboxone to help me get off of the opiates and that might be something you want to consider.
I chose to go with the suboxone for several reasons. First, every time I tried to use less I wound up using more. Also, like you, I could not do cold turkey. And I believe that because I was using it for so long and at the end at such high doses, that a long slow taper with the suboxone would be the safest and gentlest on my body. Also, it would give me time to work on my recovery in other ways including counseling, being here on SR, reading, journaling, etc.
Some people also find that with long term opiate use they develop a condition called hyperalgesia. You can look up more about it but it is basically a syndrome where the opiates begin to cause us more pain rather than relieve it.
You've definitely come to the right place for information, support and wisdom. You might want to also post in the Substance Abuse section to find others who understand about opiate addiction specifically.
Take care.