Why do i hold my mouth shut really tight i do that at night to and my jaw always hurts and grinds and pops sometimes locks what is wrong it swells to.

Seceral reasons: You may have several issues going on. You need to see a dentist who can help you. Left untreated you could end up locked and need surgery to correct the problem. There is a high probability that sleep apnea is involved. You need to see a dentist trained in dental sleep medicine.

Bruxism: Bruxism is a fancy dental term for clenching and grinding, and that is what you are doing at night. The most common reason is a bad bite (malocclusion), but some bruxism is stress related as well. You need to have your dentist make you a night guard to relieve the forces from grinding or clenching and to prevent TMJ damage.

See a specialist: Dear lethawheel59; you mentioned multiple symptoms related to jaw joint. Jaw joint (aka tmj) is a complicated structure and the fact that you are having these issues shows that you are carrying some sort of inflammation in that area. I would suggest you pay a visit to your dentist and ask for a comprehensive evaluation.

Stress: Usually nite time clenching is your subconscious response to stress during the daytime. You may need to see a doctor for an evaluation. Meanwhile, you can be fit for a "brux plate" by a dentist to limit the damage you do to your teeth and joint during the night.

You are a clencher: You are a clencher. Your dentist can help you by building a device that can balance the forces so it won't hurt so much.