Hey pjg,
I lost my dad to acute myeloid leukaemia I am 24 but I lost him when I was 4 and almost lost my uncle to the same disease last year. I am sorry that you lost your dad to cancer but your not alone it must of been hard seeing your dad in such a way I saw my uncle the same way.
I saw my uncle deteriorate before my eyes and although it was difficult I took it as the process for what it is although it was a definite struggle being so hopeless knowing there's nothing you can do.
But though my cancer journey I learnt grief is okay and that missing your loved one is normal, I sometimes miss my dad and think I am going crazy but then realise that it's fine I see photos of him and visit him at the cemetery and it's fine.
Your dad may of left this earth but he is still within you somehow remember the positive and happy times with your dad talk about these memories with your loved ones helps too what your feeling is normal and it's okay to miss him.
Be strong and brave peace xo