I think my dad died not knowing how much I cared about him
Did your dad express anything that indicated that?
No, but I feel that sometimes my dad was ignoring me before he died
Even if you feel bad that you didn't express yourself in the way you wanted, don't feel bad. I'm sure your dad knew how much you loved him.
I bet he doubted me and it hurts. Sometimes, I wonder if a parent dies and doesn't find out.
If you still doubt yourself, you can always pray and talk to him that way. You can use spiritual connection to say how much you meant to him.