YOU SAID:

I would like to have fun with my girlfriend

INTO JAPANESE

私は私のガールフレンドと一緒に楽しみたいです。

BACK INTO ENGLISH

I want to enjoy with my girlfriend.

INTO JAPANESE

私は彼女と一緒に楽しみたいです。

BACK INTO ENGLISH

I want to have fun with her.

INTO JAPANESE

私は彼女と一緒にいたいです。

BACK INTO ENGLISH

I want to be with her.

INTO JAPANESE

...彼女といたい

BACK INTO ENGLISH

I want to be with her.