More often than not, once pain is ruled out, it is the fault of the hands on the reins. Not the bit .... and not the horse.
My personal preferance is to start my green horses in a plain smooth snaffle. A snaffle means there are no shanks on the bit. Anything that does have a shank is a curb bit (doesn't matter if the mouthpiece is solid or broken). Just so we're clear on terminology.
NEVER jump to a harsher bit because the horse is not responding and/or because the horse is not listening to a bit. That will only create worse habits and cause you to continue jumping to harsher and harsher bits. You need to fix the TRAINING issue at the root.
To be quite honest, from the sounds of your post, I would highly recommend you and your friend at least working with a trainer for a few lessons. Yes, we can give you advice over the computer but it is NO substitute for hands-on learning. And just because you have the room to "do everything at home" does not mean you are capable of doing it.
Are you releasing pressure at the proper time for a reward for your horse? Fighting the bit could be a dental issue (which is why it is important to rule that out first). But more likely, you've not been giving her proper release with the bit and thus she's learned to brace against it. Best to go back to ground work and start from scratch.
Here's something to start with. Stand on her left side and ask her to bend her head to the stirrup, using direct pressure on the left rein only. Do not pulse the cue, and do not pull harder if she does not respond, but just hold it steady. The very moment that she moves her nose toward her side and releases the tension on the rein, you need to release the rein. That's her reward. Inititally, she may move her feet with this exercise. That's okay. In the end, we want her to just bend/flex her neck and keep her feet still, but its okay in the beginning if she moves in a circle. Just move with her. Eventually, she should bring her nose easily all the way to the stirrup, being soft and relaxed. You need to be consistent and always release the pressure IMMMEDIATELY when she responds correctly. If you don't, she'll learn to brace and be hard against it because she can't find release from the pressure.
Of course, do the same exercise on the right side. Always work both sides equally.
When she's mastered that, you can be "tricky" and ask her to bend to the right with the right rein, while you are standing on her left side (reach over the saddle if you are tall enough). Again, she should keep her feet still and just move her nose to release the pressure.
Do the same thing with asking her to back up. Start on the ground and pull backward equally with both reins. Hold steady until she gives you the smallest shift of weight backward. Immediately release the pressure on the reins. Next time, ask her for a small step. When she gets good at that, ask for a full step. When she's good at that, then try two steps. And so on. She should also be dropping her nose to back and not fighting the bit.
All of these exercises may take 15 minutes to accomplish .... or they may take 15 days. Every horse progresses differently. Some learn faster than others. Since this horse probably already has bad habits, she's most likely going to take longer to learn and break those habits.
Incorporate this methodology into everything you do. If you ask her to walk by squeezing with your legs, you better release that leg pressure immediately when she starts moving.
If you ask her to stop, you better release the pressure once she quits moving.
Timing is key.
Consistency is essential.
And that's why it would be VERY helpful to have a hands-on trainer for at least a few lessons. I can tell you how to time your responses to your horse on the internet, but I can't actually show you in person, which is two completely different things.