I noticed in your post that you said she "spooks at nothing." You have to realize that she is a prey animal. An animal whos first instinct is to fly from fear. Spooking is an instinctual response, and the horse won't spook "just beacue" or "to get out of work." They spook because they precieve the situation to be dangerous and that something is going to kill them. Point being: she doesn't feel safe.
I also would not put a chain over her nose, under her chin, or over her lip. That could make things worse, because pain would then be a factor, or you would just be managing her behavior, not helping her gain confidence in you or in her environment. I would however, invest in a good quality rope halter and a lead rope that is at least 12 ft. Long.
Now, the thing you need to become aware of, and sensitive to, are her THRESHOLDS. As you lead her out of the pasture, position yourself where you are at her shoulder. This way when she hits a threshold you will be able to "feel" it. You will feel her hesitation or when her body tenses up. When she hits a threshold DO NOT PUSH HER PAST IT. She is telling you she doesn't feel safe going forward anymore. Stop and wait with her. Maybe even back up away from it a few steps. You will know she is ready to go when she blows, lowers her head, blinks her eyes a lot, licks and chews, cocks a leg, lets out a deep sigh, etc. I would do this for many, many days until you hit very few, if any thresholds. Let her graze in between as well, if she wants to. This is all about building her confidence. She will respect you and trust you more if you do this consistantly with her. Good leaders do not push the other party into situations where they are afraid. Good luck!