Have taken her out for walks? this could pick her up a bit it will also give her something else to think about as the environment will be different, and maybe leave a radio on in her stable for a few hours when she is stabled..... there are also horse mirrors that you could try and as others have suggested toys. Can she see other horses? Caddy my big horse suffered severe depression when his companion died and we went and bought our Dims it was a risk because we didn't know if he would take to him..... thankfully it worked and now they have a bond you wouldn't believe!
Bless her I hope she picks up soon

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