Everything you do reflects onto your horse so try being fluid an soft while riding to start. For stiffness I would work on trotting out on a loose rein letting your horse be a horse and encourage their natural movement and don’t interfere but make sure your horse is moving off your leg. Slowly gain contact and let the horse make the decision to be soft and supple. Do not force it. Work on transitions that way your horse is responding to your leg and then encourage a frame.