Cancer-stricken Kate Middleton is putting on a brave face in front of her family, but is suffering from exhaustion, nausea and other grueling effects of her treatment, according to a new report. “Kate and William are going through hell behind palace walls,” a high-level royal courtier revealed as she and father-in-law King Charles both battle an undisclosed form of cancer, RadarOnline.com has learned. The Princess of Wales is claimed to be going through “great lengths to hide” the discomfort she has been feeling — “especially from the children.” “Unfortunately, it’s becoming more and more diff…