Serissa can take cold down to at least 50 (experiments were done on it years ago where it was allowed to go a LOT colder for ~ 3 wks and it survived, so that's not the issue. The humidity is much more likely to be the problem where you are, so try a plastic bag (untied at the bottom - you don't want mold or bugs) plus turn the pot once a week so one side isn't getting all the sun all the time. Watering it all the time, especially if it's not in a bonsai mix (and does have fewer leaves) is not the answer however.

So i have a new development.
The main trunk of my tree has a deep split in it running from just above the first branch all the way to the top.
I am guessing when i transplanted this tree i must have somehow injured its roots and the whole top dried out.
The first branch which is about 5 inches from the soil is absolutely thriving now though.
So now what do i do?
I don't want to cut the upper main trunk because it is thicker than my thumb and i think it would look stupid.