I used to have it BAD. The doctor referred me to a specialist at UW, who told me that it was the worst case he'd ever seen. I could only open my mouth 3/4".
The mouth guard helped a teeny bit. It kept it from getting any worse, which is important actually if yours isn't too horrible yet. The specialist did an MRI, which is probably what you will need to figure out how to address the problem. He could see that mine was caused by huge buildup of scar tissue from trauma to my face (probably from when I fell 30' out of a tree and broke a lot of other bones).
There's a little cartilage disk in your jaw joint that slips around when you open and close your jaw. Sometimes it can get torn off completely and get stuck, which makes your jaw stick open or shut until it loosens but it happens over and over because the cartilage is just floating around in there. In a case like that surgery may help.
Sometimes it's just simple irritation/joint inflammation from an overbite, underbite, tooth grinding, jaw clenching, etc.
In my cause, surgery to remove the scar tissue would have probably made more scar tissue in the long run, so he didn't even want to try it. There was some inflammation from the scar tissue rubbing on healthy tissue when I tried to work my jaw, and that made it worse. That's why the mouth guard helped a teeny bit. It did stop me clenching my teeth in my sleep so the little bit of inflammation would go down.
What finally cured it was scar tissue massage. A massage therapist could do it for you, and after a session or two you can pick up on it enough to do it yourself. The idea is to rub in the direction opposite the scar tissue fibers with enough pressure to start breaking it down. It's a very slow process, but it does work. As it breaks down it's reabsorbed by your body. I saw the most improvement when I used a normal electric massager thing on my jaw using the techniques my massage therapist used. It took about 3 years of that, but now my jaw is about 95% fine. Most of the time it's 100%. I can open my mouth super wide, I can hold it open, I can eat almonds, it doesn't even click anymore.
That's why an MRI is important though, because if it's not scar tissue then deep tissue scar tissue massage stuff might aggravate inflammation issues if that's the real cause. Once you know the cause of it, you can figure out a strategy to correct it.