Getting a horse to relax?

[oscarwild]

My mare was off work last year due to a liver issue. She has since been given the all clear to work again. At the beginning she had to have her ribcage freed off as it was completely locked. She has recieved physio 3 times but its now free as anything and has no further pain issues. She has had her tack checked and also had to get a new saddle. We religiously do her exercises and pole work to keep her flexible, But as soon as you ride or lunge her (lunge can be with bridle and no saddle) she instantly tenses up and wont work properly.

So I am wondering if anyone can give me any tips that may help her relax and bend and work properly or is there something Im missing that should be checked out?