Sounds like you are doing just about everything I would try.
Lots of bending, flexing, and giving when she stiffens up on you.
Ask for lots of maneuvars when she gets nervous in the pen with other horses, to try to get her to focus and attention back on you. Nothing "exciting" because she clearly doesn't need any more spunk. Maybe a nice proper rollback at the walk, or a stop and back, or leg yields at a slow collected trot.
And the more places you take her, hopefully she'll start to calm down in those environments.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.