As mentioned before, you could try having someone on her back while you lead or work with her. I usually don't have someone to help me, so I would secure a feed bag to the saddle and work with her. You could also try taking her to a grassy area and just sit on her while she grazes. Try to eliminate causes of why she does it. Not knowing her past, many things could be the reason. Maybe she was pushed or worked to hard or badly punished for not doing what was asked. Try making riding a pleasurable thing for her (sitting on her while grazing). After a while, ask for just a little forward movement and see how it goes.
I had just worked with a horse that was somewhat similar. A friend owned this mare and had pushed to hard on her. Both got frustrated and the horse started baulking, rearing, and only moving backwards no matter what you did. You could kick and whip her but she would not go forward. I spent one week on her correcting it. When she wouldn't go forward, I just made her turn side to side until I got a step or two in a forward motion. I immediately stopped asking for anything. She kept expecting to get punished or if she acted up long enough she thought she could get out of having to work. I didn't do either. Once she realized I wasn't going to quit on her or punish/get after her, she started to move forward more easily. She still has a slight tendency to go backwards but by redirecting her energy to turning, she goes forward again without much fight. It's just going to take some time. Habits are hard to break.
One last tip: Don't let yourself get frustrated. If you are frustrated, the horse will be too. If things aren't working, take a step back and work on something more simple. Posted via Mobile Device