mares witchy

I'm about sick of this routine by now. My adopted TWH mare, Jubilee, is 12 years old and has been both a show horse and broodmare. After getting her in shape nutritionally, my sister and I started trying to progress in riding her. This is what I find strange... - Jubilee is perfectly...

I had three [3] mares that were living happier ever after. then my wife suggested we either ride more or sell. In order for her to ride more, she needed something more her speed. We went out and found another mare that is quite nice. She's about 20 years old, but you wouldn't know it by looking...

Ok, so the last one I did, I really didn't get a response to my question... most of you put that your mares get really witchy when they are in heat, but when my one goes into heat she is super nice, but when she isn't in heat she's really witchy.. so would spaying her really do anything?