I think we have similar problem. Most of third generations of chinese in Indonesia dont speak mandarin anymore. There used to be joke "chinese that dont speak mandarin probably come from Indonesia" due to our government discriminatory policy that forbade Chinese to speak, write, or read anything related to Chinese during the past.
Even though I born and live in Chinese family, we hardly speak mandarin with exception my father and his cousins who still speak chinese. My mother herself is able to speak dutch, it's pathetic that none of these languages inherit to me.
Even though recently I have begun to develop feeling of proud being Chinese and having plan to study my ancestor culture in future, I dont have any plan to study Mandarin for meantime because I heard it's very amazingly difficult.
I also dont live in community where there is a lot of chinese speakers around, so learning it now is totally obsolete. I guess Japanese and German are enough for me.
Anyway omnislash, I think you should try not forgetting your mandarin by keep using it everyday instead of english, because you have chance to be bilingual somehow =P.
Just like you, I have friend that has fluency over british english since she was raised in England during her junior high school. Now she is totally clueless when she must write in Indonesia instead of English.
Jam it back in, in the dark.