I don't know if you're already using 'pressure and release' but it's worked wonders for me with a pony that was fearful of water.
The concept is that you ask for the movement you want, and the
instant they give you any kind of a move in that direction, even at first just a slight stretch of the neck towards you, you stop asking. Then repeat repeat repeat. And do it all again the next day and the next and the next. My Fell literally walked into the puddle of his own volition - before I even asked - on the third session!

It seems to me that once they realise that yielding is pleasant (that gentle but insistant pressure on the lead rope stops) and nothing scary happens, then their natural response to pressure becomes to yield rather than to fight.
Tessy is lucky to have found such a caring home, I'm sure she will continue to grow in confidence
