Horse has your number. It doesn't suit him to ride off, so the holes in his training show up.
And the flash does nothing. Gaping mouth means either you are IN his mouth too much, hands aren't still, or he is using that to keep from moving off.
This is rider respect problem, not equipment problem.
Imagine it is showing up elsewhere too, as it usually does.
You need to get this horse paying attention to you ASAP.
There are many threads on here for dealing with barn sour horse, which is what you have.
Horses make me a better person.