I'm so sorry. I know kind of how you feel. My grandmother, to whom I was very close, died 4 years ago on Friday. SHe had been very ill and I was her primary care giver. Even when you know it is coming, it doesn't soften the blow. I can only imagine how it feels to have your father taken so suddenly.
I wish I had a magic answer to make it feel better, but the honest truth is that it won't. You need to know that it is ok to be upset and cry and do whatever it is that YOU need to do for you.
I'm guessing that your friends and FI just don't know how to handle the situation. My now husband had no clue how to be around me. I was really angry with him for that. He's a "fixer" and when he couldn't fix me, he withdrew. His friend who is doing so well might not be. Some people put on a show or it doesn't hit them for awhile. I think it took me about 2 weeks to really come to grips with the situation, I was trying to be strong for everyone else.
I would suggest finding support somewhere. A priest, a bereavement counselor, an online group, anything. It usually helps me to write a letter when I'm upset. I usually don't send them, but it helps me express and process what I'm feeling. Write to your dad and tell him what you feel, whatever that may be - hurt, angry, alone, sad... Know that it will take a while to get better, but it will get better. If you have to take things one hour at a time or one day at a time, do.
Know that we will be here for you in any way we can. If there is anything I can do please PM me. My heart goes out to you. *Big Hug*