There are a few reasons why your pastel pencils might be breaking. The most likely is that you're over-sharpening the nibs. So try using a light touch when sharpening your nibs in future and avoid exerting too much pressure on them. Also be aware than standard pencil sharpeners can be too harsh, so use a knife instead, or a specialist sharpener designed for larger cores. Finally, high temperatures can affect the nib's integrity so make sure you store your pencils in a cool, dry place.