I'm not sure it is what you are looking for but the mayor of Bamberg; Johannes Junius was accused of Witchcraft in 1624 and sent a letter to his daughter pleading his innocence. Transciption available here.

Whilst it isn't a diary of an actual devil worshipping, spell casting, witch of superstition is does given an insight into the last moments of someone accused of witchcraft in the 17th century.

If you are looking for someone who was a folk-healer, cunning-folk or wise-women rather than those accused of making covenant with the Devil. Then understand that these sort of people would have been rural, poor and illiterate, therefore any first hand source would be unlikely to occur.

In regards to published journals; Have you looked up any Grimoires? Though many of them are the product of occultists and often clergymen rather than the classical concept of a witch. Though the term witch can be interpreted in many ways.