Do not ever think about falling or you will tense up too much. Are u having proper lessons cos if so they will teach you how to position you foot in the stirrup so you wont get caught up, also they shoul provide u with safety stirrups if u r a novice. Concentrate on having as safe a seat as u can then relax and enjoy it. U are more likely to unbalance when u r learnig to trot properly. Enjoy yourself its a brilliant, healthy sport.