Show soon, need jumping help!?

Ok so I have this arabian cross, she is is the only horse I own that can jump but she is terrible at it, she pops the jumps so a 18 inch jump feels like its 3 foot and no matter how strong my jumping position I always get tossed around and nd up ripping some of her mane out, and I fel terrible when i do that, shes just a sweet old mare, does anyone have any tips for a stronger position, smoothing her jump out? It would help alot because I'm taking her into a hunter hack show and I have to jump 2ft :S