Grief is kicked off any time our lives are changed in a way that was unexpected. We suddenly become aware that our life is changed and cannot go back to how it was - maybe before you had children, after a relationship breakup, even a favourite manager leaving at work - all of these mean we are forced into accepting a new reality. We therefore have the grieving process, as we are sad for what has been lost, maybe shocked, maybe angry that the change has happened and you are out of control - and finally some acceptance that life is different but can still be good.
Add to this grief over a loved one, and it is easy to see why that feeling is so overwhelming. We are already coping with a world that keeps changing on us and then all of a sudden there is another change - and it cannot be undone.
So give yourself a break if you are trying to come to terms with the loss of a loved one and finding it sometimes just too much. The process takes however long is needed, and the more you try to suppress it and ignore it the longer it takes to get released (and it WILL get released - it doesn't go away it just festers). Allow it out during your quiet moments and take as many of them as you need.