I helped a student with similar problem a few years ago. If I understand your description correctly, this might help. Put the heel of one hand on the sternum at the best angle you can manage, trying to keep the fingers off the chest wall. With the other hand, grasp the wrist of the hand on the chest and use it to help the first hand push down.

The new thinking is that a lot of uninterrupted compressions (100 between ventilations) do the most good. Ventilation isn't being stressed in the way it used to be, so push away, pal.

As Jim Lad says, you may well break something in the patient's chest, but that's often just the price of compressions.

I've been a first responder for 24 years now, and breaking ribs has certainly happened to me, and every other old medic I know.

Here's what I remind my students: We do CPR on people who are pulseless and non-breathing. That's pretty dead. You can't hurt the dead, so get on with doing the best compressions and ventilations you can.

D