Oddly enough, I just use my girlfriend less.

(On a more serious note, 6-month girlfriend fluency in Japanese is something I've observed a few times, and it's a great stepping stone but mostly just barely functional. Especially when your situational choice of e.g. verb form broadcasts so much about your educational background)

putting oneself in a position where you interact with a person whose first language is not English does tend to increase the odds you will learn a second language provided both are interested in that outcome