When a patient breathes through their mouth, the tongue drops down towards the lower teeth to let air into the mouth. This causes the teeth to part slightly and the mouth to open. Without firm contact between them, the teeth over-erupt, because they naturally tend to keep growing until they meet. This process can lead to the face becoming longer.
In addition, mouth breathing leads to a number of other effects, such as: a narrow upper jaw, a high, dome-shaped palate, a wide lower jaw, an open bite at the front teeth and a crossbite at the back teeth. The teeth also dry out, making them more prone to decay, and viruses and bacteria enter the body more easily, increasing the risk of frequent illness.
These changes most often appear in childhood, while the jaws are still growing. If breathing is corrected in time and air starts to pass mainly through the nose, some of these unfavourable effects on jaw development can be offset and minimised.
This answer is for general information. Your own case needs an examination by an orthodontist.