My best advice to you is simply bomb them with it. Don't let them make you feel ashamed for what you believe nor push you to the side because their religion comes first. I am not saying become an extremist and hurl it in their face every chance you get, but, fight fire with fire. If they bring up their religion when it is unwarranted, then bring up yours. Never feel shame, never feel like you have made a mistake. Druidism is an honourable religion. If it comes to it, let them know you are an adult and have a mind and belief of your own.
Personally, I am a pagan in a family full of Christians. I know how you feel, Christianity never felt right for me, even more so when it was forced upon me. The irony of my side is that my grandfather was a Christian man yet he encouraged me to look back to the ancestral gods of my people. We Ásatrú here in the north are a proud lot of folk, there is no reason you should not be a proud druid. I remind my family every time I see them by dressing with Ásatrú charms on my belt. It is not to be rude, it is to be defiant and rebellious and free. It's to show that there is more than one belief in the world, some much older in these lands than others. It's also to show that I am a separate thinker, I have my own mind and my own ways apart from theirs.
I guess what my point is, is that you ought to be strong and unwavering. Just because their religion says one thing does not mean it concerns you. If your father has an issue with you being Druid, or therefore your own conscious being, then let him deal with his problem. After all, it is him who has this problem. Clearly you have no issue with being of the druidic faith. You are a grown woman now and he must accept that you have beliefs different from his own. In the end, you must do what makes you happy, but don't wait too long or it will begin to weigh on you. It has been 4 years since I last saw or said a word to my father. He too has a "holier-than-thou" mentality and it is something I will not tolerate. So I chose to sever all ties. I am not saying that you have to do the same because you don't. But do what makes you happy. Let him vent his words of condemnation if there are any, let him throw a fit if he does. In the end, you still are you and you still have your beliefs and that is all that matters.
Not all may share my beliefs of what I've written down here, but, that is of no concern to me. This is simply what has worked for me. It's not to be militant or extreme, but to be proud no matter what and to push back when one has been pushed.
Blessings to you from a northern pagan and good luck in it all.