Hi.. Welcome to HCM.. As per your complain severe pain on clenching teeth is due to excessive continuous pressure over the jaw joint and along with it there is wear and tear of the jaw joint along with injury and damage to the supporting ligaments and tensed muscles of the face.. Clenching habit is generally seen in individuals who have excessive stress.. As you wear mouth guard at night and not in day then it can be effecting your joints during the day causing severe pain.. I would suggest you to first of all consult an Oral Physician or an Orthodontist and get evaluated and a proper mouthguard will be fabricated that will help in reducing the problem.. Along with it a combination of muscle relaxants like Tizanidine and painkiller like Ibuprofen can help.. You should also get be advised Physiotherapy.. Also consult a Psychologist and get evaluated so that he can do proper counselling and advise methods to reduce stress and if needs medications can also be advised.. As of now take painkillers like Tramadol to relieve pain.. Do deep breathing exercises to reduce stress.. Take sufficient rest.. Hope this helps.. Regards.