Don't worry. Everyone grieves at their own pace and in their own way. At my age, I've seen a lot of people go, and every one is different. My grandmother and my aunt were sisters, and I loved them both pretty close to equally, but when my aunt died, I was devastated; when my grandma died, I was just relieved. See, my aunt went quickly, and it seemed she was snatched away from us, while my grandma had a stroke, and the parts of her brain who made her who she was were pretty much destroyed, so it seemed like peace at last. That may be the case with your mother, if she had a long decline as you describe. Don't beat yourself up. It will come in its own way, and no matter what way that is, remember, no one mourns the cocoon after the butterfly has flown.

All the best,
~ Jack "Blimprider" Tyler