that is wayyyy not enough time to learn. You need to practise higher than you show- im planning to go to a show and I have to jump a D grade and ungraded competition because my horse is D graded, so that's a starting height between 90cm and 1m10m, and I've been told not to go if I can;t confidently jump 1m20 with him at home.
I've started 1m20, and trust me it is a HUGE difference to 1m10,altho its only 10cm
I throw my heart over the fence, and he follows it. ~Sir Louie VI- my showjumper, my best friend, my everything-iloveyou.