Firstly, a stunning conclusion! I loved it.
Just a few grammatical errors, but overall its good.
Suddenly dad's lecture that once sounded nagging and monotonous became very important. They were his final gift to me,
If you chose they as you have done, then the earlier sentence should state lectures and not"lecture".
Not until I'm older, I came to realized
Here check out for the tense..." Not until I was older, I came to realize".
The loss of my father has changed me. I could no longer talk to him, but I can call out to him through my actions hoping he will see.
Tense! "The loss of my father has changed me.I can no longer...."