Complex sentence: In his last letter to his wife, written on the day he died, Liddell wrote of suffering a nervous breakdown due to overwork, but in actuality he was suffering from an inoperable brain tumour; overwork and malnourishment may have hastened his death.
Split into simpler sentences: In his last letter to his wife, written on the day he died, Liddell wrote of suffering a nervous breakdown due to overwork. He actually had an inoperable brain tumour; overwork and malnourishment may have hastened his death.