Brother, I'm in the same boat. I joined the same year Clinton took office and ETS'd the last year of W. Four deployments in the 90's, and half again as many from 9/11 until I was med-boarded out for back injuries on my next-to-last deployment. Had three disc fusion surgeries (ACDF) while on active, so my medical addiction started even before my EAOS. I made it through my last deployment in '06 with a bottle of Lorcets always on my person - all I had to do was go to the corpsman whenever I needed a refill. After I got home they referred me to a civilian "pain specialist" who kept me so doped up it was silly. I didn't hurt mind you, but damn if I could function. After getting out and starting to be seen by the VA, they kept me on the same dose at first, but thankfully I had doc who would listen and (at my request) eased my dosage down over about a 2 year period so I'm at a level I can function at - 60mg of MS twice a day and 1800mg of Gabapentin twice a day. After taking narcs daily for something like a decade now I've got a ludicrous tolerance, and 120mg a day of Morphine doesn't really do squat but take the edge off - but that's all I really want. I'd rather live at a pain level of 3-5 (depending on the day - the weather, how I slept, etc etc) and be coherent.
As far as living in the present... you couldn't have said it better about having lived an entire life in the past. I joined at 18 and got out in my 30's (I'm just hitting 40 now). About 85% of my adult life up till now was spent on active duty - and in those 16 years I did more, saw more places and met more people than most folks do in a lifetime. The absolute hardest part of the whole thing for me - besides feeling like a junkie at times because I need pills just to get out of bed - is to let go. Not forget, but not let it drive where I'm at now, or where I need to be headed - and that's damn hard. Its hard because it's (for the most part) all I know. Its hard because NO ONE gives a crap - other vets are the ONLY ones who can even remotely relate. I've started going back to school in an attempt at a second career, but all my medical issues make it real challenge. I feel totally rudderless most of the time. It may sound cheesy, but there's a quote from Sitting Bull (it's actually a one-line song he wrote while imprisoned at Ft. Bufort) that I first read about a year after I got out - and it rings so true that it almost brought tears:
"A warrior I have been. Now it is all over. A hard time I have."
You're definitely not alone.