HI
@RetiredUSCG
There are a few things that will cause tinnitus to increase after a long habituation period of 10 years or more. Although you don't know what caused the tinnitus, something is usually responsible for its onset. The first is exposure to loud noise and there are various kinds that I will explain below. The second is an underlying medical condition, within the ear and auditory system or elsewhere in the body, including TMJ as you've mentioned. Stress has been known to increase tinnitus too.
If (USCG) is an abbreviation that you are a retired United States Coast Guard, then I suspect that your auditory system would have been subjected to loud noise at least some of the time, requiring you to wear hearing protection? If you developed tinnitus while doing this job then it's likely your tinnitus was originally noise induced? Even if you didn't experience tinnitus during service as a USCG, it is not uncommon for one to develop it later in life as a result of previous noise exposure, as some musicians do after they stop playing music in a band. Therefore, it's something to consider.
Prior to your tinnitus increasing three months ago, were you a regular user of headphones, earbuds or a headset? If yes to using any of these devices, then they could be the cause of the tinnitus increasing. Headphones/earbuds are common causes of tinnitus and often responsible for increasing it after a person has habituated and begins to use them. Tinnitus can also be caused by an underlying medical problem within the auditory system. I suggest that you see an ENT doctor so tests can be carried out. Hearing loss can cause tinnitus to increase as one gets older. If the tinnitus is TMJ related, I advise that you see a Dentist/Orthodontist so your jaw can be checked as you may need to wear a mouth guard.
If tests at ENT reveal hearing loss or noise induced tinnitus, then you may need to be referred to an Audiologist for tinnitus management that can include hearing aid(s)
Please click on my Avatar and choose "started threads". In the list look for the posts: Tinnitus, A Personal View and The Habituation Process, that you might find helpful. There are other posts.
All the best
Michael