I brush relatively regularly, use mouth wash but grind a lot.i wear braces. recently back teeth ache when brushed &when clenching jaw.1 looks cracked?

Normal ,but: its normal to have teeth ache during braces treatment ,as they move from pressure by the braces , but if you think you cracked a tooth ,you need to see your dentist ASAP as that might lead to complications .

Bruxism: is characterized by continuous clenching and grinding of teeth. Bruxism can easily wear down your teeth. If you suffer from bruxism and are worried it is damaging your teeth, a personal consultation with your dentist or orthodontist is the first step in finding a solution that works for you.