Agreed with NM. Definitely take the dam out of the equation. She has to focus on YOU not what mommy dearest is doing.
As for breaking her. I find that breaking a horse to ride goes better with small tiny steps. We have harness racers so harness are readily available. What we do is put a harness (without the crouper or buxton). It is light but gives them the feeling of "something is there". I find especially with fillies, that they become girth sour very easily. Just take it slow and easy.
Start but put the harness of loose (if it slips it doesnt effect too much since nothing gets in the way) and lead her around. Continue this until she seems fairly comfortable with it.
I wouldnt suggest using a western saddle, it looks like a big monster ;). Use a small english saddle with no stirrups. Begin lunging her so she gets the feel of something larger on her back. This is the part where you also introduce the bit. Use a simple snaffle with a regluar bridle. (I apologize if Im repeating things you already know
Once this process is complete and you feel she is comfortable with the whole situation its time to put something on her back. Now I suggest having someone assist you at this part as you mentioned you are intimidated by her. If you have someone around you it subconsciously should quiet your nerves a bit lol.
Mount her and lead her around (no steering on the part of the rider let the person on the ground to the steering at first.
All this depends on the reaction of your filly. I've worked with large horses and the worst ones get rammy which can be very irriating and intimidating. Once again I apologize if I've repeated things you already know just trying to be helpful :)
Update us on how your coming along with her!!