Psychiatrist Elisabeth Kubler-Ross has explained the 5 stages of grief. It's not necessary that one would be going through each and every stage nor is it necessary that these stages would be experienced in the same order. But the final stage would be 'Acceptance'.
During a loss, there is no explanation as to why things had to turn out that way. Be assured that you will be able to get over the pain through time and the intensity of grief will diminish. Don't beat yourself up by thinking worse things that won't help in any way. Ensure your grief does not lead to depression. Let each day pass, face your feelings, if you feel helpless, that's okay too. Because as each day passes, a self healing process is taking place, without your knowledge. You will know there is a difference from the way you feel later compared to the way you feel now. And a day will come, when you would be able to put everything that happened behind you and appreciate the good memories and good times associated with the person or situation you were grieving over.