Dear Gorak,
All planets can influence a person’s life both as a benefic or malefic depending on their placements in the horoscope chart. There is nothing like Mukhya Grah or Ishta Dev. All grahas are equally important and all devas are ishta devas. If they were not good, they would have been called Asuras. These are all terms used by wayside astrologers and does not mean anything.
Do read my articles on http://www.navagrahastro.blogspot.in where such myths and misconceptions are well explained as to their futility.
Your 1st, 2nd and 10th house are rather weak in your horoscope. That is what is giving you problems in career. For a detailed scientific evaluation of your horoscope please come through Paid services.