You don't need a lighter bike, you need more practice. Racing is a combination of fitness, technical skill, and to a lesser degree, tactics. You need to build up your anaerobic threshold as part of your fitness plan. Unless you are an incredibly smooth jumper, and can manual the rythym section easily, you need to spend time working on those skills as well. It also helps to study your opponents. Some guys just suck if they start from between 6-8 off the gate. Some may like to pass high or low, or on a certain turn. Improve your game first, not your bike.