How long does it take for a horse to adjust to new bit? and the problems with old bit

Some informations:
14years old mare, well broke, was a showjumper before, now just trail riding (for 3 years). Her teeth are floated, was ridden with 21mm (0,83inches) single joint snaffle for 2 years (when I had her), then one year with 16mm (0,63inches). Her previous owner sometimes rode her with bit with no joints, 21mm, but mostly with single joint. Don't know with what she was ridden before... The last owner had huge problems with her, mare was throwing her head up on shows and ran away from the bit, owner couldn't control her and they never did good. She also used other things that helped her to hold mare's head down...

Problems:
She is always opening her mouth when I'm bridling her, she doesn't close her mouth until I go out of the stable. I ride on loose reins, she is very good, when I gave her a signal on loose reins she starts to chew on the bit (I was told that's good though), but if I don't have loose reins she opens her mouth widely and moves her head on a side. She also does this if she is nervous, scared, or bored (doesn't matter if I have completly loose reins). If she is bored I start doing something with her and she stops, but if she is nervous it's impossible to make her stop.
She always had both nose bands (I don't know how are they called in english), to prevent her from opening her mouth, they didn't help. I took them both down last month, no changes in her behaviour.
I came to the conclusion the bit I have is too big for her, that she has a small mouth and maybe low palate (?) and the single joint bit isn't the right choice for her. I bought a french link bit yesterday, 12mm (0,47inches). She was a bit better when I rode her, she deffinitly didn't open her mouth so much, but if I took a hold of reins she did it like always. Maybe it's a reflex, a trauma of all the years she wasn't ridden with a right bit?

Is it possible to re-train her, so she wouldn't open mouth so much?
Will it go away by itself if this bit is ok for her and how long do you think it will take?
Should I be doing something else too?
What else do you think could be causing this problem?
How should I ride her?
Do you think it would be good to ride her with non-loose reins, so that she will find out it doesn't hurt (if my guess is right, then single joint bit was making contact with her palate and that caused her pain, so she opened mouth. With french link this won't happen. But if my guess isn't right, than holding reins will cause her pain and I don't want that)

Any advice or your story will help. Tell me your opinions please :)

Just bumping this up...

How tight do you have the bridle done up on her?

If it was me, i'd go back to a snaffle(i prefer a fullcheck snaffle) she how she does in it then progress fwd. Id make sure she know how to give with whatever bit you decides she needs or likes. It may take a while to find out for sure,but lots of flexing will help you be able to decide how she takes to it..

Two wrinkles. I'll take a pic tomorrow and put it on.

Thanks for advice jwells84.

I was riding her today for the third time in the french link bit and I have to say she did better in it, especially today. I didn't ride on loose reins so much as we are used to and she opened her mouth when I shortened the reins. But after few steps she closed them and was ok. Well every time I did the transition from long to short reins she opened mouth, but closed it very very soon. I'm quite sure the snaffle was a problem and it bumped her in the palate.
I'll work on flexing :)

I had a gelding that did this also that I just sold. He did best in a myler bit. I contacted myler and showed them videos of her, ect. They recommeneded a bit and I got a used one off ebay. Worth a try.

Try lowering the bit a little one wrinkle

My general rule of thumb when I was playing around with bits on my horse, was if he didn't seem to get along with it in one ride, I need to try something new. Thankfully, I've invested in a lot of bits so I always had plenty of options to try.

I agree with what jwells suggested. Go back to square one. Start back at a snaffle (I tend to stick with a medium twisted wire snaffle). True plain snaffles don't offer enough on a truly broke horse. My both my show horse now and my old show horse from when I was a youth kid were fussy with their bits. Wound up my old show horse was a dream in a myler that had about 8 joints in it. Phenomenol bit. My show horse now, even though he's 8, can't stand any kind of port or breaks in a bit. Just a basic mullen type bit keeps him happy.

You know when you get the right one.

I really don't think it's the bit that is causing your horses problem. Horses adapt very quickly to things that they can't change. In my opinion your horses is expressing outward signs of inner termoil. Like when a person chews thier fingernails or constantly drums thier fingers. There are likely many holes in your horses training that need to be filled in before he will feel like he can calm down and trust you.

When I get a horse like that I work on softening them and getting them to give to pressure. I want them to break at the poll when I pick up a rein and give me vertical a lateral flexion with relative ease. More impotantly I want to be able to direct the horses feet and control the life in them. This means I have to be able to move each part of the horse independently.

An excellent book to read is "True Horsemanship Through Feel" by Bill Dorrance and Leslie Desmond. He talks at lenght about feeling your horse and letting your horse feel of you. When you get that feel happening really well then they can feel when you want them to be calm and stand and you can feel when they just HAVE to move and can't stand any longer so you direct them where you want them to go.

It took your horse a while to get like this and he has been like this a while so progress will be slow but it can be made. Look at what your horse has going on inside his head not what is going on inside his mouth and you will have much better results.

I just changed my horse's bit yesterday upon discovery that his previous one seemed to be causing him some discomfort. While he immediately did seem more comfortable with the new one, we walked him on contact in hand for a good half hour before he was convinced it was okay to reach into the contact. Once he realized that all was good, he was fine with it. If you ride your horse normally on a loose rein, I'm not sure how exactly you figure out how much your old bit came into play.