I moved my UserCP navigation at the top to span horizontally rather than vertically. That solved my problem. However, I'd use a conditional in your postbit template such as:
Add to the very top in postbit or postbit_legacy template:
Code:
<if condition="in_array(THIS_SCRIPT, array('private'))">
Your PM postbit code here...
<else />
Add to the very bottom of your postbit or postbit_legacy template: