We always try to end any horse session on a positive note. If she is dangerous to be on her back, go ahead and just do some ground exercises until you get a good response then put her up. You don't want her to think if she is being a stinker she gets out of work.
By the way Vida is 14.3 and I am a fat broad at 5'8. I know I look huge on her but we don't get too strenuous and we take lots of breaks.