[YorkieMother]

I not going to pull any punches here cause it not going to help her in the long run if I do.
You need that girl to see a vet behaviorist right now!!

What you are seeing in the chasing is obsessive compulsive disorder and this can be caused by seizures in the brain we can not see and it is not going to be helped by anything but drugs.
I even go as far as getting a hold of Dr Dodman Director of Animal behavior at the Cummings School of Veterinary Medicine at Tuffs University and see what he thinks.
I suggest this as light and shadow chasing is his thing and he was the one that found out many of these guys are having seizures so tiny that they can not be seen but it makes them do this light chasing against their will.

Taking the wall back by standing between it and her will not help if she is having medical issues it will push her to find a no OCD and she will find another way to do it when your not around and they do.

Please go find someone that understands OCDs and that it is a medical condition needing drug support and not a need to run it world... she is doing this because her body tells her too not cause she can stop it and it nothing she or you can do or did that made this happen. Something is not wired right in her little brain and it can be rewired with the right help.
I be cranky as all get out if my body told me to chase lights and I could not stop I be barking at and grumpy with others too.

Vet run and one that is trained at a very high level this is not for trainers or anyone else it got to be a specialist. As you go play with trainers they will think she out to take over the world.
You go get help from a vet that does not understand they will drug her into a stupor that will not help either.
I am sorry if I made your heart jump and you are feeling worried and afraid... these guys can and do live a much happier life once treated the right way and it will be just fine it will take time.

JL