This is is my mare, but in the extreme. She has zero concentration she is very spooky and is too busy being in edge and I guess looking out for her life lol, yes our outdoor school poses a threat to her life. I find it so hard sometimes getting her to listen let alone go in an outline.
What you need to try is the following ;
A) you must have your horse off your leg and forwards (this is no' 1 key)
B) every time your horse fixates on something turn in opposite direction
C) with horses like this, you have to be busy and consistent with them. So transitions (not the odd one here or there, a hundred transitions - transitions within the pace too that includes.
C) don't wander around on the track, ride using the while school lots of movements - so much so they start to listen as not sure what's coming next. Use pole (not necessarily three trot poles but pole scattered around, poles to circle around, poles shaped like a square ride over, through, around etc you get the idea... But remember if and when your horse looks or fixates go the opposite way. Good luck