Ground work question

  You're most likely too strong with your body language.  Soften your shoulders, your eyes, take a half step back, put the stick down, stop asking 2 steps before you want her to stop....   They will all lessen the amount of pressure you are putting on her.

Can you put her next to a wall or a fence?  That way if she tries to turn away she'll be blocked. 
For the neck flexing I did this in a few steps.  I saddled him up everytime.  First lesson we used a rope halter.  I held the saddle horn then pulled gently on the cheek piece.  Holding the saddle allowed me to stay with him when he backed up or spun which he did ALOT during the first few tries.  once he tried to flex his neck I let go of the halter.  He caught on fairly quickly. 
Lesson two we did with the rope halter and lead rope.  I pulled on the lead rope (still holding the saddle horn) once he tried to flex I gave slack.
Lesson 3 we did with bit and bridle.  I pulled on the near rein gently (still holding the saddle) once he tried to flex I let it go. 
After each sucess I asked for a bit more flex until eventually he was touching his nose to his shoulder (this is a hard feat for the thick necked fjord horse).  We just went nice and slow and only worked on it a few minutes a day. 
Now he flexes his neck wonderfully with only a light flutter of my fingers on the rein.

If she's running you over with her front end, don't forget to block her.  On the other hand, make sure she isn't reacting out of fear.  If you can, the next time you ask her to yield her hindquarters, observe what her face looks like.  If she's wide eyed, and her ears are back, if her tail is tense or in a "J", she's scared and you need to back off immediately.  Then just do a lot of petting and scratching in that zone until she's comfortable with you back there.  Ask a little at a time as soft as you can and be really slow with your phases.  Reward the smallest try - that means even a shift in her weight! - and then stand up immediately and smile and rub her.  Don't forget either that if she's scared, she needs time to think.  Wait til she licks her lips or puts her head down and blows.  Build on how far you go slowly (1 step, wait, 2 steps, wait, etc.).  If she's not scared, then you need to take a big breath and have her move until she offers to stop.  It's kind of like, "You want to move faster? Great idea! Let me help you!"  until she says, "Oh. Gee, you're good at this game!  Can't we just stand still for a minute?"  LOL - I've had to do this before, and it gets them to think twice before they try to take over leadership!  It proves to the horse that you're calling the shots.  But make sure she's not scared!  If you do this and she's scared, it'll ruin her confidence and her trust in you as a leader!