Especially as you've only had her a few months, I'd say this is most likely a relationship/trust issue. Especially as she is OK in company(security of a herd). Horses are very perceptive to movement.... could be a camouflaged predator hiding under one of the flowers!
I would be first spending time with her, in evironments/situations that she is comfortable with, to build her trust & respect for you. Then I would do some desensitising to movement, in those comfortable surrounds, before finally taking her out & about alone, to test & reinforce the lessons.
Also depends how you're handling the situation & her when out. Eg. do you think you ride any differently when alone? Less relaxed, more on guard yourself?(Given her behaviour, I'm guessing you're starting to, even if you weren't at first) Do you ride with a strong contact, or do you allow her to relax on a loose rein? What do you do when she reacts? Try to push her on, let her turn & run, allow her a minute to check out what the monster really is??
It would depend on the level of her fear/reaction as to what I'd do(assuming I felt we were up to that stage in my relationship with a horse). I would allow them to stand & look for a minute, then I would relax & ask the horse to pass the object, so long as there was room for her to pass at what *she felt* a safe distance. I wouldn't try to force her too far from her comfort zone. If she were too reactive or there wasn't a comfortable amount of room, depending on the situation & what we were up to training-wise, I might ask her to just go a little closer, then turn back, repeating this process until she was OK with going further/closer/past. I might get off & lead her past the scary, with me on the inside. Or I might just ask her to turn & walk away, come back to it another day.