I started with C++ as my first language and then branched out to Java and C#. Later I learned less similar languages, like Python.
Going back to C++ of course sometimes you will need to use reference a little more than normal, you're not going to "forget" it.
Think of it like playing a game at the arcade, and then playing a game on a home console port. You don't forget how to play the arcade game just because you are playing the home version. Programming is the "game" and the console or control is your "language." If you play at the arcade again after playing your home version for a long time, you will of course have to get used to it again but you will not have forgotten how to play.
EDIT: It was said, but your goal is not to memorize a language. Even professional developers of many years still use language references. They don't have the whole language memorized inside and out. As you get experience you will of course memorize more and more syntax, but you'll never know every last thing and retain it fully. And it's much more useful to be familiar with many languages than to know one single language at such an arbitrary level of perfection.
The skills you gain by learning C++ will transfer to C#. C++ and C# are very similar, and at your level, you probably won't even notice much difference at this point.