Looking at the pictures , sorry to say but your bridle dosen't fit your horse.
The cheekpieces are too long ( not the cause of the bridle coming off ) but this is not a major problem
I would like to see the browband longer - this would enable the headpiece to sit further back - really shouldn't be touching the ears quite so much.
Also the headpiece itself isn't right - the flat part between the splits should be longer - at the moment what is happening is that the split between the headpiece and the throat is too high and is forcing the browband higher onto the ears - which is why your bridle is coming off.
You need cheekpieces that are about 3" shorter, a browband that is about 1.5" longer and the distance between the splits on the head piece should be about 2" longer ( approx only as working from photos ) Basically the browband and headpiece are too small.
The bit looks to be sitting OK so no major probs there - could do with sitting a fraction lower but if your horse is happy with it there keep it that way.
Hope this helps