Hi, I understand what you are going through. My mom left when I was 2 and remarried twice. And my granpa dies a couple years ago and I was pretty close to him. It is very hard when major changes like death, marriage, divorce happen in your family. You may try to talk with a school psychologist or counselor. I am older then you but the pain is pretty much the same. If you can not find someone at school to talk to there are also hotlines you can call if you are really upset. Look on the internet and find a hotline in your area for something like teens in crisis or just plain crisis hotlines. I hope you can get some help with dealing with and coping with these major life changes. I will be praying for you and I hope I helped a little.
I would suggest talking to someone like a trusted adult, a school counselor, or a therapist who specializes in grief therapy. It sounds like you need someone to talk this through with. You've had some pretty major events happen over the course of a decade for your age. A divorce during your formative years, remarriages, and now your mother's getting divorced again. Not to mention losing your grandfather. I lost mine two years ago to congestive heart failure, so I know how you feel. He was very special to me. It's hard to say goodbye and let go. But I do know this: Don't hold it in regarding your feelings about his death, your mother's divorce, etc. Please find someone to talk to.
In the meantime, I suggest journaling about your feelings and focusing on finding out why you feel the way you do.
Hang in there!