Fear with Jumping High?

Okay, I asked this before, a long time ago. A couple years ago I jumped around 2'7 but that was only a couple times in my lesson. Later in that lesson I had to jump a high crossrail and vertical line.My trainer told me to trot out,but of course, I'm on an TB/GW ex-race horse so of course he cantered out. After that I lost my stirrups somehow and then I found myself holding onto his neck while he hand gallops around the arena. I finally fall when he slows down. I lost my confidence that day jumping higher. Of course I am now more confident, I have been riding for six years. I am a 10th grader and want to learn to jump higher comforably before i graduate. Can you help me?