Mind-shield (Manomayakosha) is the barrier we form as children to protect ourselves from unpleasant feelings like hurt for example. This is because we believe that everything – including those feelings – is real. Once we realise that the reality is pure consciousness and what is not permanent is not real, this barrier of Manomayakosha must go making way for the bliss to shine. Manomayakosha acts as a barrier preventing bliss to shine which is our essential nature. It is okay to be somewhat vulnerable to feelings once it is known that the reality is non-dual. Even if self-realisation has not taken place, but just this understanding takes place that fears are behind many acts that we unconsciously do and it is those fears that prevent us from experiencing ever present bliss, we are ready to work towards diluting the shield of the mind (Manomayakosha) via self inquiry. Diluting Manomayakosha via self inquiry means finding the root cause of any feeling that arises which is of the nature of hurt, anxiety, approval seeking, low self-esteem etc.
PS: Fear is not the only component of Manomayakosha. Desire is also another major component. The above write-up concentrates on the fear aspect.