Your headaches, jaw tension, and cracking are all symptoms associated with Temporomandibular Joint dysfunction (TMD). Your oral appliance looks very similar to a tongue crib, which is used to correct a tongue thrust. Has your orthodontist diagnosed you with that? You said that your tongue is pushing against your teeth, so you can breath through your mouth, yet when I asked if you have allergies or asthma, you stated you didn't know if it was important. It's very important. Long-term habitual mouth breathing means your orofacial muscles are behaving dysfunctionally. Can you breathe through your nose? Has any therapist told you how you should rest your tongue? What therapists have you seen? Oh yes. I've had experience with many of your issues. These were many of my issues: jaw tension, cracking sounds when I yawned, eating too fast, had braces twice (first time relapsed, then had a permanent retainer glued to my top 4 teeth), persistent headaches, horrible childhood allergies with persistent headaches, problems concentrating, and stuttering. I've only recently recovered from stuttering, so everything I've learned from my journey is fresh in my mind.