I don't want to speak girly Japanese.

[John_nato]

Recently I started dating this girl who I speak with in japanese. She has a really feminine way of speaking and I think it might be rubbing off on me, and changing the way I speak.

I don't want The little Japanese that I have to become some 可愛い女の子みたいな喋り方, でもね、アメリカに帰ってから、あんまり日本人と話さないし、会えないし、もうできるだけ日本語を忘れないように、この人と毎日話してますが、自分の話し方が少し変わってきたんだ。どうすればいいんですか？

例えば、 I think I am speaking in a high pitch more.
I notice that I almost use words like かしら。
I think I am starting to laugh like her too...

[Javizy]

Making sure you're aware of the nuance of things like かしら, のね etc is one of the most important things. JPod had quite a few lessons on gender-based differences. You could probably find a book on the topic too. If you're already aware of all the differences and struggling, then I guess you just need to balance the exposure you're getting with your girlfriend with exposure to other kinds of Japanese. That could include TV, films, manga and speaking to other Japanese people (try Shared Talk). Exposure is important in developing a sense of what sounds "right", so it's only natural you'll start sounding girlie if that's your only input.