Complex sentence: Eventually, Livia is almost driven mad by the fact that she is unable to see Franz and rejoices when a letter from him finally arrives, thanking her for the financial support that helped him stay away from the front.
Split into simpler sentences: Eventually, Livia is almost driven mad by the fact that she's unable to see Franz, but rejoices when a letter from him finally arrives. In the letter, Franz thanks Livia for the financial support that helped him stay away from the front.