Start by going back to your basics. Work on introductory dressage skills like bending, shortening, lengthening, etc. Take lots of lessons without stirrups and practice your two point without stirrups. Also, do lots of exercises that you enjoy and that you feel confident doing. Essentially, focus on skills that strengthen your foundation as a rider.
After you spend some time doing that, usually after a couple weeks or so, then start working your way back up to where you were when you fell. Push yourself a little as you're progressing, but not so much that you get discouraged and quit. The goal is getting you to feel confident on your horse--keep that in mind every time you ride to help you prevent getting discouraged, too.
Everyone is different and it may take you a short amount of time or a long amount of time to regain your confidence, but it's most important to take it at a pace that's comfortable for you. Hope this helps!