amastrike -
Breathe. :yes:
Ok, so, here’s the thing - like i’ve said before, I had a filly very similar to yours. Only reason i think i didn’t have nearly as much trouble as you are having is that mine was born with a keen, enthusiastic attitude toward people - she wanted to work, she wanted to learn new things, so she rarely ever gave me grief (once i laid down the law and showed her who was boss - at 1 day old).
But, every so often, she would plant her feet and refuse to do something. And i really thought it was just her being difficult, because of her demeanor when she did it. There was no typical “i’m scared” mannerisms, just a very matter-of-fact “nope, not going over that and you can’t make me - muwahaha”. One day, we encountered our first bridge on a solo hack (i think i’ve told this story before…). She was SO determined that day that she would NOT go over that bridge that she actually backed up, in a straight line, about 30 steps. :eek: :mad:
So, i didn’t get mad - i got even. I called my BO, she came out with her truck on the narrow dirt road (dead end country road), and BLOCKED the filly from backing up. Filly hit the truck, shot forward towards the bridge, and my BO inched the truck forward. Filly stopped, backed up a step, hit the truck, shot forward. You get the picture.
All of a sudden a lightbulb went off, and she was like “oh crap, i’m actually going to have to do this”. And THEN she got scared. She arched her neck, she snorted, she pranced, and finally she scooted across the bridge. Was never an issue again.
This filly was afraid of very little, and i could also toss anything on her back and she couldn’t have cared less. But just because she was fearless in nature, didn’t mean that once in a while she didn’t have little fears about things (she was deathly afraid of pigs the first time she saw them - but minis? small children on bikes? cows? pff - no problem).
So you said in your post earlier today that when your girl finally went over said ditch, she did it quickly. Just like mine, she gave in and scooted across it. If she wasn’t at least a little bit afraid, she would have just given in and been all nonchalant about it and walked over it calmly. But she didn’t - she scooted across it because she was a little bit nervous, for whatever random reason horses are nervous about silly stuff we don’t understand. The reason she didn’t act scared at first is because she had no intention of going over it - so why be worried? 
So, i would agree with Kyzteke, ditch the chain, get a long dressage whip (and if you hold it in your left hand with the lead rope in your right, there should be no awkwardness and the whip should not “get in the way”) but rely more on your body language and voice. And keep doing what you did to get her over the ditch - she goes forward, praise. Another step, vocal praise. Wait her out.
You have made your point re: you being alpha, you have seen her demeanor change around you - you say she’s still balking every so often, but i’m curious - has she nailed you again since you laid down the law?