It's not always about your predicted grades, people seem to misunderstand. It's about your current AS grades and the potential to achieve the required grades alongside many other factors. You have to sell yourself to the admissions team and in this case unfortunately you were not good enough. If dentistry is the only path you want to pursue you may be able to apply through clearing (which may or may not be possible, I'm not sure) or take a Gap year and apply with grades in hand. This way you can match your grades to the ones required and consider whether or not you're actually capable of being admitted onto a dentistry course. I'm sorry to hear about your rejections but keep your chin up and move forward. You can't re-roll the time, just make good out of a bad situation. The most important thing is you've learned from your mistakes.