The sudden onset of these problems really sounds like a health or pain problem. If you have ruled this out then it is ok to do some retraining. Horses rush because they are scared or bored. When she does this, the best thing to do is softly circle her until she is balanced and slows down. Just pulling will make her unhappy and make the problem worse. Do all of your work on circles and serpentines until she can preform transitions calmly. Then you can move back to asking for straight lines. Good luck!
Only perfect practice makes perfect.