My dad died 14 years ago after a 31 month battle with cancer. I can relate to you as I also watched my Dad as he went from a strong and active carpenter to weighing just over 120 pounds. My dad and I were never, what you would call close. He divorced my Mom when I was 27 years old, remarried and I saw little of him for many years, although I tried to keep in touch. My daughters never really knew him. His wife wanted nothing to do with me or my family. I never doubted he loved me, regardless.
When he was diagnosed, had surgery and the cancer was found to be all over in his torso, his wife couldn't even tell him. A distant relative called me about my dad having surgery that day so I drove and arrived in time to talk to the surgeon. I told my Dad he was not going to live much longer. I was very lucky he was determined to live because he stayed with us for two years longer than expected. I took that time to spend every minute I could with him even though I lived a days drive away. I was with him the day before he died and promised him I would see him in a couple fo days. To this day, it kills me I wasn't with him when he died. I can't seem to forgive myself for not being there. Enough about that.
You will never get over the loss of your father. He is embodied, in part, in you. The weirdest things, a smell, a sound, a song, driving by a hamburger joint, something you say without thinking ,will trigger a memory of your Dad. Grab hold of that and cherish it. There are so many kids who never knew or will know their father or have the blessing of a father who is there and loves and nurtures them. We who had our Dads and lost them are the lucky ones. I had to do a repair on my house yesterday and it required some tricky angles and measurements before I cut the materials. It went perfect. I used some of my Dad's tools during the job. When I was done, I had to say "thanks Dad" out loud to myself. You taught me how to do this.