hey, yeah i sometime feel myself stiff up when i approach a jump, but most of the time its because i get excited- i love to jump!- and yeah, but when i first started to jump my trainer would lay down a pole in front of the fence to clue in with the horse when to jump, so i could concentrate on other stuff, but now i still dont always get it right, i either get left behind or get up too soon.
but yeah, you know how you said you dont know when to take off? it would help to count the horses strides/steps and kind of in your mind measure out the distance from the starting point to the jump . . . it sounds kinda confusing, so sorry, hehehe.
but i juss wanna say that a really bad thing to do is look at the jump or the ground while you go over, because you might loose everything, try looking over and past your horses ears if your just doing a single jump, but if your doing more try to look more on towards the next jump
and to help with the pulling on the mouth thing, try to grab hold of the main when you go into the jumping position . . . hope i helped, and if i didnt and just further confused you with the way i put things. . . sorry, hehe