How can I?

It’s so hard for me to wish my aunt and uncle a Happy  Father’s Day or Happy Mother’s Day for the simple reason that although they’ve been my legal guardians for years, I can’t actually see them as my parents. My mom died and I don’t know my dad’s whereabouts. It’s not fair to think this way but I can’t help it. I know they’ve done so much for me and I should be grateful but I just get so many conflicting feelings. I can’t understand why I’m like this and I just hate it.