Is your number of logged dives and your number of years certified accurate in your profile? If so, that'd probably explain the jaw fatigue. I'm not trying to be critical... my first year of diving, say 35 dives or so, my jaw hurt for a day after diving and I went through a mouth piece in that time. My next mouthpiece lasted me maybe 150 dives and my jaw didn't hurt. I've only used maybe 2-3 mouthpieces since and I'm pushing 2K dives. I'm using a 3.95 mouthpiece these days.
ScubaSteve001 (same first name, but we're unrelated) is exactly right... relax... and not that that's easy to do. It'll come in time if you get some frequency. Most people initially bite down on the mouthpiece, when all's you really need to do is just keep your mouth somewhat closed. I'm not sure that spending extra $ on a fancy mouthpiece will fix the fatigue overnight, it might help, or it just might be that you might be making the purchase at about the time you start to relax on the bite.
Good luck, and keep divin'.