Nicole, I truly feel your pain as I sit here in tears. This brings back a lot of memories. I'm so sorry this happened to you, and your feeling that you're going through this alone. Not a comfortable place to be. Losing a parent no matter what the situation is a great loss, which can leave a void in our lives for a long time.
I grew up living in orphanages with my twin sister . We had a mom and dad that didn't want us. I lost my identical twin sister when we were 11 years old. I suffered horrible unspeakable abuse from my parents and grew up in deep pain, until I went to counseling. It took 8 long years to get through all the issues that had been inflicted upon me as a child and young adult. We all learn to cope and somehow survive terrible tragedies in our lifetimes. Everyone has issues, but it's how you deal with them that makes all the difference in your life. No one ever said life is easy, but surviving these tragedies will only make you a stronger adult. Focus on the wonderful memories you had with your mother, the good times. Don't focus on her passing, this will ease the pain. You will always remember this time no matter what you do. The pain you are feeling now will become less and less as time goes on until it is gone. This is how our brains cope.
Think of the loss like this....you now feel empty that your mom is gone leaving this void in your life. I actually saw a hole in my heart. I filled this hole with love, visualizing it this way. I would grab the word love bold and in all caps, hearts, and anything else that signifies love in your mind and put all of it into your heart filling it up like you would a basket (mental imagery). You have the memories which no one can ever take from you, and that's what you hold on to. In my case I had to let go, as I realized you can't change yesterday, but we hold the power to change tomorrow.
You are now old enough to say who you want to live with, and your father is where you should stay. Your grandma and grandpa will understand and obviously love you. Keep the lines of communication open, and when you're comfortable use that time to talk to your dad and his fiance about what you feel. They are adults and will understand your feelings/needs and support you through this. Just let her know that you're not ready to talk right now, and you will let her know when you are. Maybe for now you can confide in a trusted friend or teacher.
Trust me that there is a Grand Creator (God) who is watching over you and will bless you with the strength to pull through your loss.
Take care my friend.