Trying to find a specific book's name, can anyone help?

I've read a book a few years ago where there are these two countries, one is really cold and one is really hot

The countries are at war, and the people from the hot country have magic while the cold country have good technology

The main character grows up in the hot country and ends up in the cold one, I think because she was conscripted and sent as a spy

Because her body is acclimated to the really hot country she can't stand the cold and has to use magic to keep herself warm (seem to remember her using her long hair to channel it underneath a wetsuit at some point

::: spoiler Potential Spoilers The main character ends up falling in love with a soldier from the cold country, and as far as I recall they end up getting shunned by both :::

I'm fairly certain someone I know would love this book but I can't find it for the life of me, does it ring any bells for anyone?