my recommendation would to have a chat with the uni to see if they can help you, as already suggested by MMM.
secondly, if your boyfriend will work in japan, perhaps the company he works for can help. if he won't work so be it.
the third alternative would be to start living separately in the housing provided by the uni, and then when you have arrived start looking for something else.
although it is possible to find and get access to a flat in japan within a couple of minutes, i am not certain that this applies to you for a number of reasons. you are a student, you are foreign, and perhaps the most important thing, you are not present physically yourself nor does it seem that you have a proxy.
good luck.
here are some of the more common sites for flats in japan, all in japanese though as far as i know.
賃貸・不動産情報はCHINTAI [賃貸マンション・賃貸アパート・賃貸住宅・賃貸物件な ど不動産賃貸の部屋探し]
www.apamanshop.com
賃貸情報フォレント[賃貸マンション、賃貸アパート、賃貸住宅、賃貸不動産 の検索・情報サイト]
www.mast-net.jp
賃貸のレオパレス21｜賃貸アパートやマンスリーマンシ� ��ン、一人暮らしの物件情報
ピタットハウスで賃貸・売買・不動産を『検索』 | 賃貸・売買・マンション・アパート・中古マンション・ 一戸建て・土地
minimini.jp
賃貸情報エイブル [賃貸物件・不動産物件を全国から検索！賃貸マンション ・賃貸アパート・賃貸住宅情報サイト]
www.eheya.net
̒徳島の賃貸情報「住もう」