Your feeling of betrayal is understandable given everything you've explained. However, there's one thing you left out: time.
A lot of time has passed since your youth. You no longer hold the same religious views as you did back then. It's safe to assume that your dad's views have also changed, though perhaps not as drastically. This likely includes his views on the citizenship ceremony. Today, there are options that were once to be avoided at all costs. But you, your dad, and even his religion have changed.
So why the feeling of betrayal? Perhaps it's because your dad hasn't admitted to you that he was wrong back then and that his views have since evolve?
I don't know about you, but I don't believe I suffered significantly from not celebrating many holidays and other things. What made it an excruciating experience for me however were those who bullied me, including adults! I was constantly ridiculed, hit, spat upon, and my things were vandalized — all because I wouldn't celebrate Christmas, date girls, or stand up for the national anthem. Today, even though I celebrate those holidays and sing the national anthem at hockey games, none of that changes the fact that those kids and adults who bullied me were absolutely reprehensible, far more than my religion and my parents who raised me in it.
Today, I look ahead and don't spend much time thinking about who did what wrong and how long. You're parents are probably getting older and if they still talk to you despite being disfellowshipped, you know what? Take it as a blessing. People don't have to ask for forgiveness for you to "get over" things they've done. Just see your parents as flawed individuals who, despite all their flaws, still love you and always did.