I am a Gulf War I vet, I felt the same as you when I returned home from combat. For me it was the total sense of feeling alive and being apart of my squad that I missed, although I did not figure this out until after 15 years of insanity.
In the mix, blood pounds through the veins and I received a powerful sense of completeness that I still chase today. The intensiveness of combat will never be matched in the civilian world, all the mundane things we did before seem totally a way to piss us off today. When faced with survival we let all the silly shit slough off of us and become one with the universe.
Our field of vision opens completely to encompass all within our sight, the tiny reflection in the corner of the eye becomes a sharp focus without having to direct attention its way. Time becomes suspended and we know and feel what omnipresence really means. How can anything else ever compare to this experiential endeavor?
I finally received help after 15 years, I could not drink enough alcohol, smoke enough weed, or seek out enough violence to get past the feelings of emptiness. I felt such an utterly and complete loss of self and sense of identity. We were trained to feel invincible, and it may even have seemed that way at times, but we did not get through combat without the help from the soldier next to us. We cannot make it through the confines of our minds without the help of others. We could not do it alone in combat, what makes you think that you can make it alone today?
Follow the links in the side bar and see how I came through to the other side (under the heading "A Suggested Guide to PTSD Management).