horse will not take bit

Question
I us a D ring snaffle. and he does great in it, after I finally get it in.  I just started trying a rubber covered D ring. to see if it might be a sensitivity to the metal. No, I do not change a lot.  I want a mild bit on him. he is a very sensitive horse,  I consider my self very light handed also.  I do not pull or jerk on his mouth.
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-----Question-----
Hi, I consider myself a very experienced rider. I have been braking and training horses since I was 13, I am now 41, I have shown, trained , broke horses all my life, but I have ran into a problem... I have a 7 year Arabian that I have been riding since he was 3, he has never liked the bit being put in.but wouold, but now it is almost impossible.  I have placed black licorice in my hand so when he takes the bite, he gets the bit.  I have done the Clinton Anderson technique of putting a rope in mouth , just in case it was the medal he doesn't like.He refuses it too.  I have had is mouth checked by a vet. and I have changed to a rubber bit. I still have a battle..  I do not fight him, and he will drop his head and not fight, he just will not open mouth.clenches teeth. not even with thumb in mouth.  if I take the bit away and put my thumb in, he will open instantly,  but the minute he feels it against his lips he refuse, I can even put my whole hand in his side mouth and he just refuses to open. as for getting it out he will just drop it out calm. no problem once it is in. Any suggestions
-----Answer-----
Laura,
 What type of bit have you been using? and how long have you been using it?  Do you switch bits alot?
Samantha

Answer
Laura,
 I will almost garauntee that the snaffle is pinching his tongue, and that is why he doesn't want to take it.  I would try a straight rubber coated bit, or try riding him in a hackamore/bosol for awhile to let him relax, and not worry about anything in his mouth hurting him.  I bet you will see a world of difference.  I had a saddlebred who I rode hunter/jumper with and he also had a very sensitive light mouth, and I also ride with very light forgiving hands, and I could not figure out why he would not put his head down, and he would get so upset.  Well, he had cuts and scars all over his tongue from that snaffle pinching him.  I put him in a hackamore for  6 months, then into a straight kimberwick, and he was a different horse.  I know kimberwicks can be harsher, but with a very light hand, you will be fine.

Samantha Brunner