I'm afraid my answer is this : Start her entire education over.
Sometimes you can "patch up" training holes and carry on with a horse, but sometimes it's best to just wipe the slate clean and start again.
This starts with earning her respect - and if you're nervous around her, maybe this can be helped with a good coach who is good with groundwork (you might be surprised how many good riding coaches seem far more lost on the ground). This is more about YOU have the confidence to lead your mare than it is much else.
If you're ending up in a stand off in the round pen, chances are you're needing some eyes on you to help you out a bit, because somewhere there is a block coming from you to her - and she is showing it to you by saying "NO".
Once you've earned her respect, and trust, then proceed to "restart" her saddle training. Treat her as if she's never done it before (not with nervousness... but as if you have no "expectations" based on past experiences).
For me, this means we work on the long lines/do some ground driving, make sure we have no tension during saddling, do some work from the mounting block (getting her to side pass up, then stand quietly while you touch her, lean on her, mount her etc). When that's all relaxed and "easy"... THEN I get on and ride.
Generally, if you make yourself aware of tension in your horse, you can learn to diffuse the situation before she ever feels the need to "explode". It takes practice... and it really does help to have GOOD eyes on you (someone who can see and recognize when tension is building - before it results in a truly tense horse).
Remember that your energy (nervousness, excitement, anger, frustration, happiness, relaxation etc.) will immediately be transferred to your horse, and the horse will reflect that energy back to you however it knows how (and that will based on their past training). Strive to only allow yourself to feel relaxed and open minded (it takes a lot of practice) and just try to make your only goal having the horse reflect THAT feeling back to you, no matter what training you're doing.
We train our horses everytime we spend time with them - for better or worse.