Welcome to Barnmice Della!

I had two mares at one time, I could never turn them out together once my gelding died as the Paso Fino mare spent her time beating the Arab mare up.  If there was a livestock panel between them they would cheerfully stand together, but loose in the same enclosure the Paso Fino beat up the Arab.

I've never had a horse afraid of wide open places.

All my horses sort of changed on me, I now think they may have gotten EPM or some other neuronal parasite, this was long, long ago.  My first horse, totally reliable on road or trail started bolting at cars and trucks.  Formerly sweet mares (to other horses) turned into viragos.  I had not idea what was happening, I hope your mare does not have these pesky nervous tissue parasites, but you might want to consider/test for EPM (a parasite carried by opposums that prefers nervous tissue) or Lyme's disease (a neuronal rickesetta carried by ticks.) 

I do know that as I moved from mostly open pasture to more wooded areas it seemed to get worse, but I am not really sure about this since my undiagnosed Multiple Sclerosis was hitting me pretty bad back then messing up my memories.