well my bottom jaw never grew as quick as the top my allergy is to pollen
There are a number of reasons for mouth breathing. The first thing to consider is your nose. Do you have allergies, a deviated septum, or enlarged turbinates? These are issues that your your doctor can address. Dr. Mehregan's comment is also right on the mark. Most people who are chronic mouth breathers have jaw structures that aggravates tongue collapse, especially when you're on your back, and especially if you're in deep sleep, due to muscle relaxation. Having smaller, narrower jaws also narrows your nasal cavity, which also makes you more susceptible to nasal congestion due to anything that causes inflammation (such as allergies, colds, etc.).
There are many good reasons for breathing through your nose: the nasal mucous membranes make a gas called nitric oxide. It not only acts as an antimicrobial agent, it also increases oxygen absorption by 10-20% when inhaled into your lungs. Think about what you're missing out on when you're constantly breathing through your mouth. Also, you probably don't get deep efficient sleep due to the mechanism that I described earlier. Having nasal congestion can only aggravate this problem.
thanks also I can't sleep on the floor because it cuts off my breathing what could cause that
Airway obstruction, tongue placement to name a few. You probably should consult an MD first.