I’ll re-iterate the advice; no jeans! Wear some cycling liners, they do help. See if you can borrow some chaps (the lower leg only type) as they will help prevent your trousers rubbing against your legs if you try trotting. Boots with a heel are best, if you wear shoes/trainers the stirrup may work its way over the tongue/laces of your shoe and sit against your leg which is uncomfortable, and dangerous as if you fall off you may get dragged if your foot wont come out! I stopped wearing my steel capped work boots for the same reason, they were too wide for the stirrups and wouldn’t come out easily if I fell off.
Never ever lean back if the horse decides to go under a low branch, always lean forwards. Hug the neck and put your head down one side of the neck if you really think you are going to get taken off!