Hi, I am (were) a Bonsai spacialist. A diagnosis from a far is very difficult but I see the main problem. Your Bonsai isn't a indoor Bonsai. As you can see from the links the others sent you, this is a mediterrean plant and so the tree would normally have a "light" winter rest wich means the tree should have a palce with a tempratur between 5 to max. 10C. and a light place, in this time of year. In many cases it is a problem to provide this kind of location. And watch out while the tree tries to become a state of winter rest it woun't need the same amount of water while the whole cycel is slow down...
I hope this might help you in some sort of way.. I'm dyslexic and english is just a second lang. to me, so I hope you'll get my key message of this reply. I'm sorry in advance for all mistakes I but down in words

(grammer) here.
If you have any more quest. don't hesitate to contact me.