Relax. I have taught people who were so tense I could have snapped their joints!!!
Ask your instructor if someone could lead you on the horse, take your feet out of the stirrups, rest your hands on your knees and let yourself go 'floppy' and sink into the horse rather than 'perch on top of it'
When you feel the tension go put your feet back into the stirrups - just on the ball of your foot and work with your arms outstretched for a while. Ideally people should learn to stay balanced at the trot without reins before they are allowed to hold the reins at all.