I am so sad that your dad was taken so early in life. It breaks my heart.
I lost my sister 13 years ago, and it took a while for me to move on with my life. She did not die from a horrible cancer - she was taken from us very quickly in a car accident, which I definitely think is easier than what you have gone through. I was also 17 when it happened, and left for college the following year. It haunted me for a long time. I found that therapy definitely helped me to pick up the pieces and try and move forward. Through constant effort over a period of time, I was able to move forward. But this didn't happen for a good 5-6 years, although it did not seem that long when I was going through it all. I still have bad days, but for the most part I am happy when I think of her. I eventually stopped thinking of her death when I thought of her; instead, I only remembered her life. And with some of the memories will come a bit of sadness for not being able to form new memories. But I can honestly say that I am so thankful for the 17 years I was able to have her in my life - her presence helped shape who I am today and I always laugh when I do something that is characteristic of her. I like to think of it as her living through me.
My friend was recently diagnosed with this cancer. As a result, I contacted a former co-worker who lost his wife to breast cancer. He told me that my friend may want to join a support group. Support groups and therapists are not for everyone, but I would definitely recommend giving it a try if you haven't already.