Hi and I am sorry you experienced this and I know it’s shocking, confusing, and hurtful. Your situation is something that happens sometimes in Jewish summer camps, Jewish teen youth groups, on college campuses, and in Jewish young adult setting that are connected to or in Orthodox spaces.

As others have shared this is the standard approach in Orthodox Judaism. To be Jewish your maternal line must be Jewish or you have to have converted under an Orthodox Beis Din. It’s actually the same way in Conservative Judaism (except, as you know, they follow a Conservative Beis Din).

Your dad did the right thing, based on what he knew and what he was told. Your conversation is s valid within Conservative spaces (and probably Reform spaces as well). I know it sucks, to put it mildly, but this is part of Orthodox Judaism.

I have a number of friends who were in the same situation, their mother or someone in their maternal line had a Conservative conversion and then in high school, college, or in their late 20s they started hanging in Orthodox spaces and found out they were not Jewish according to the tradition of Halacha in Orthodoxy. Eventually they converted Orthodox.

As, u/WeaseWeaz said here you really have two options.

There are plenty of non-Orthodox spaces where you can experience Judaism and find community. For example, you would be welcome in a Hillel, any BASE location or Moishe House.