In my experience with this topic, I feel that addiction is much related to wanting to control the feelings we have, to change them according to the way we want to feel. It seems like you asked yourself "Why am I doing this" (like when you mentioned that tonsil pain was gone, but you used anyway.) By the way, my drug of choice was ETOH, and although I suffered extreme health complications (no legals- thank God) I drank anyway. I was preoccupied with it 24-7 and enjoyed the feelings that you described using opiates. And those feelings are entirely real, wheather they came from the drug itself and it's direct effects or the feeling of control from being able to escape from what you are truly not wanting to feel. Sometimes to me, it was just that I'd rather not feel at all, then I could get out of myself and survive on that numbness and never get hurt. Trust me, this I understand.
You have a family history of pain (abuse, addiction etc.) so yes, there is a good chance that your body was chemically set up to produce a high that may or may not be the same effects that others feel when taking certain drugs and drinking. Nonetheless, I think that if we are aware of these things and have been informed of certain predispositions, then we have a choice (like was stated in an earlier entry) as long as we are sober/ out of our addiction state (not using, or in a mental relapse or whatever). I personally had to do the 12 steps, make daily "gratitude lists" (even if they are just little notes to self in my head) and constantly remind myself to do the "next best right thing always". You'll be amazed how this takes the focus off of you and reminds you of how lucky you are to be you, which was something that I forgot during my addiction.
The other thing I grabbed onto is something I read in one of the posts above... "once an addict, always an addict". It's true; I found other addictions, (although less destructive) that I engaged in. Saying this, I can guarantee that if I had allowed them to be, they could have become as destructive as my drinking was, and I luckily had the knowledge and the experience of my AA group (and God) to help me recognize them and halt them in their tracks.
They say that our use is "but a symptom" of our disease, and I truly believe this. I heard one guy say at a meeting: "My problem is me, not the alcohol, I am addicted to self". I thought he was crazy at first, but it is entirely true. When I am focused on me and what I want constantly, or in a chronic state of the "poor me syndrome" I am unable to see the real issue. I am of little use to myself or anyone else. It's our way of self protecting, just like when we studied mental illness in school- (not that we are mentally ill) but we fall somewhere on that continuum of disease and when we are sober, we have the ability to beat it, or let it beat us.
It sounds like you have taken a lot of positive steps in your recovery and are seeking out help, even by writing this post. Don't ever underestimate the power of God (higher power or whatever) and his ability to work through other people to help you. I believe addiction is a spiritual malidy, and a psychological state of ill health with physiological symptoms. So wheather of not that constitutes disease, I am grateful today that I can talk to others about my feelings, and that I have the choice to do the "next best right thing". Good luck to all who have come through this and chose to live life as well as those who still suffer.:)