My mother also passed away 10 years ago when I was 12. The concept of death is a constant nagging horror in the back of my mind that won't go away. I've never been able to make myself feel any better about it, and I probably never will. I guess the only consolation is that, ready or not, it will eventually just come and then I won't worry about it anymore.
I'm terribly sorry that you had to go through losing your mother at 12. My older brother died a little under two years ago, when I was 18. Like you, death is ever-present in my mind. For me it manifests as anxiety of losing my parents, sisters, and friends. I'm not worried about my own death, though I am morbidly curious as to what my final moments will be like. I hope that even though you don't feel any better about death, you find comfort in other aspects of life.