Target heart rate is just one way to measure the intensity of your workout, and doesn't always work well for everyone. My suggestion would be to try a different method, such as the Talk Test or Rate of Perceived Exertion. Here's a link with more detail on those: www.sparkpeople.com/resource/fitness_artic
As long as you're working hard and challenging yourself, that's what is most important. Try not to focus too much on specific numbers.
Hope that helps,