# I have not concealed your righteousness in my heart

"I have openly told every one about your righteousness"

# in my heart

Here this refers to the writer's inner being. 

# I have not concealed your covenant faithfulness or your trustworthiness from the great assembly

"I have told everyone in the great assembly about your covenant faithfulness or your trustworthiness" 

# your covenant faithfulness

"how faithful you are to your covenant" 

# your trustworthiness

"how trustworthy you are"