I have seen others here in the past in exactly this situation,as Emz said when you are a child you don't yet understand what is missed. you miss them of course but you tend to think more day to day at a younger age..As a young adult when you start to think about the future,educational achievements,careers,marriage,kids or whatever those choices are for you,then you start to think much more how sad it is for you and your dad that he didn't see these things or won't see them in the future,and naturally have some jealousy of friends who do have that.
I think bereavement counselling might help as this is impacting your life now,then now is when it is likely to be most use,and as Emz says writing here too,as when you are writing in a way to explain your feelings too others helps explain them too yourself.