My mare plants her feet and refuses to go forward?

I have a 13 year old mare that I have owned since she was 5. She was green broke when I got her, so I worked with her and an instructor to teach her training level dressage. She really did well. She is the type horse that needs to be worked 4-5 times per week. After a couple of years, I put a western saddle on her and we rode miles of trails together for the next couple of years. Then another mare had a foal and she seemed to get jealous. It really changed her. The foal would call out to her when we went to leave the property and she began refusing to leave the property. Around the same time, I got a job where I traveled a great deal and could not devote 5 rides a week to her. When I would ride her, she would fight me leaving the property as though she was barn sour. i am an experienced rider, I know how to cue her to move forward, but she refused. If I tapped her with the crop, she still would not move. If I turned her to move her feet, she would back up and then abruptly stop. Very frustrating!!! But eventually, something would "click" in her mind and she would take off at a trot and we would have a great ride. Then, last year, another foal was born and she got jealous again. She bonds to those babies! I could not get her to leave the property with me on her back. I would then take her to the equestrian park, lunge her and then mount; the fight would start all over. Today, I took her to the park and she planted her feet and simply refused. There was another rider there that was willing to ride with us, but that meant nothing to her. I put spurs on and she did work out in the arena, but the minute we left the arena and headed for home, she refused to move forward again. I tried and tried. I should note that I was riding her with a rope halter. She has been very responsive in the past and I can ride her at any gait with either a bridle or a halter. I am so very frustrated and I really believe that there is some tie to the foals, what do you think? I love this mare (She is a maiden mare) but I am beyond frustrated with her at this point. I am open to any suggestions! Thank you! Cindy