My horse panics when his head is tied. He will pull back until he's free. Is there any way to correct this? From what you told me it sounds like your horse was never properly taught how to be tied. Your horse sometime in the past learned that if he pulls hard enough the thing holding him will give and so now he keeps pulling until something breaks. The good news is that it can be cured but the bad is that it may take a bit of time.
I assume your horse leads well for you. If not make sure he can before you try the following.
First off we need to teach your horse to give in to pressure. Stand in front of your horse with a lead rope that is at least 12' long. Lightly hold the lead rope in one hand and start to pull the rope with the other hand but let it slip through that hand as you bring it closer to you. Now switch hands and then when you let it slip through this time hold the rope a little tighter in the hand that is letting the rope slip. Continue switching hands and gradually keep adding a little bit to your grip until the horse starts coming to you. As a safety precaution so you don't get a rope burn, wear gloves and if your horse starts to panic and pull back allow the rope to slip through your hands till you get to the end of the rope and then if he still is backing, walk with him until he stops. Then at that time you walk backward holding the end of the rope with him following you till you get back to the point where you started.
Once your horse consistantly comes to you by just lightly tugging the lead then we can start working with a hitching post. Using a lead that is about 20 feet long have your horse stand about 12 feet or more from the hitching post. You then will stand on the opposite side of the hitching post and practice having the horse come to the hitching post like what you did in the previous paragraph.
When he comes to you without hesitation when you tug on the rope we then will have you on the same side of the hitchen post as your horse but have the rope go from the horse to the hitchen post and then back to you. Both of you will be about 4 feet from post. Now lightly pull the rope towards you which will cause tension in the rope to the horse. Ideally we want him to step towards the post and walk past you but if he doesn't but instead backs up then drop the rope so he doesn't feel any pressure. As soon as he stops pick up the rope and then lightly pull it towards you again. Keep doing this until you can get your horse to walk to the hitchen post whenever he feels the pressure you put on the rope. You then ought to be able to tie him to a hitchen post without any problems. You may have to practice this same scenerio with a tie on your horst trailer or anything you normally tie him too.
Note: Whenever you do tie him make sure the rope is at least long enough where he can reach the ground with his nose. This way he won't feel so nervous and scared because he will have free movement of his head.
Work at your horses pace with this. He may do fine with just an hours worth of time or you may have to work with this for several days. In any case have fun with your horse