To properly transfer the weight to your hips, you will need something stiffer than a pad. Proper weight transfer requires either metal stays or a stiff plastic back, each of which needs to be connected to the hip belt. Load lifters can also help a bit, but the structure of back stays connected to hip belt is of primary importance.

There are plenty of lightweight hiking backpacks which will do the job. I'd ditch what you have and start over. Your back is worth it!

Visit a store that carries hiking/backpacking supplies (such as REI) and take the books with you to try on packs!

I only wish that more students would take this step, which will prevent a lot of injuries.