How can i teach my horse distance ?

My mare and i just started jumping, she has never done it before so im trying to teach her . So far, after a year, i made it to 2' verticals. but the thing is, she doesnt think she can make it without charging towards the fence. She ends up jumping flat and throwign me off balance. its because when she gallops towards the fence (just so you know, when this happens , she grabs the bit and is almost uncontrolable... just so you dont blame me for evverrryyything) Her strides lengthen and when she gets to the fence, she either jumps supperrrr early or jumps in another very short stride. What can help me with this ?