Just a tip from a rider who herself has gone through a number of dips in confidence. Give yourself time, try have a person you trust (more experienced rider or instructor) that can encourage you with you when you ride.

I've had two really big dips in my confidence with my riding and with both dips it helped that I had (or changed to) instructors that encouraged me and knew how to push me out of my comfort zone just enough to build my confidence. Try and put a neck strap on when you ride in english tack, mentally it helps to just know you have that safety net.

Take it slow, start just walking. If you have access to a lunge ring try riding in there at first and slowly build it up. This may sound stupid but don't forget to keep your shoulders back, heels relaxed and BREATH. Talk to your horse or if you don't get embarrased easily sing, talking and singing actually forces you to breath.