Everyone’s responses are very pessimistic and mildly patronising on here?
Mildly fatigued I'd say, from again answering someone who started to investigate how to take care of a bonsai AFTER purchasing it.
I'm not pessimistic, by the way. Seems your tree just has a serious case of autumn.
Did I misunderstand you have a greenhouse? Why would you want to bring it indoors if you have one? Even if you don't, why bring it indoors? No tree has evolved to live indoors. Unless when you can give it the climate it needs, it's just not a good idea to keep it there.