There are three significant points here. First, it's only been a few months since your grandmother has died, and you have not fully processed her death. Be easy on yourself. A flood of emotions is normal. It would be helpful to talk about your loss and your memories of your grandmother with loved ones, caring friends, or a counselor.
Second, people grieve differently and take different amounts of time to process the death of a loved one. Your mother and grandfather likely have had more experience in dealing with loss in their lives. This may be the first significant death experience for you, however. Your mother and grandfather may have been bracing themselves for her loss mentally, emotionally, and practically so that when it actually came it was less surprising. The support they are receiving as a daughter and widower may be more substantial. Alternatively, they may be withholding how they are really coping for fear of upsetting you. Try to open up about how you're still having trouble dealing with it. Ask for their love, time, and patience. You may be surprised.
Third, because you each had different relationships with your grandmother, her passing had different meaning for each of you. Consider in particular what you are specifically angry about. Lost opportunities, for example? Is there something you can do to help bridge the gap?
I'm sorry for your loss and hope that you can use this loss to reconnect with your family and remember your grandmother's memory. Don't hesitate to seek the assistance of a mental health professional if you feel you aren't processing the grief well in about a year.