You have bought a soft toothbrush, but is it cleaning your teeth gently? The bristles and design need to be of high quality, so they do not splay out while you brush causing damage to your gums and enamel.
Many people are brushing their teeth too hard and using toothpaste that is extremely abrasive. Did you know that most 'whitening' toothpaste uses abrasion to make your teeth whiter?