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.
Just as taste is hidden in food, bliss is hidden in Self. Just as taste itself cannot experience taste, bliss itself cannot experience bliss. A contrast or comparison is required for such experience to take place. Self needs to become many from one to experience what is present in itSelf. Thus, duality is born out of non-duality. Self projects the world and plays the role of living beings to experience bliss and to experience itSelf.
Experience of bliss increases as Sattvaguna becomes more and more prominent. However, as Gita says Sattvaguna binds by creating attachment to bliss (refer this). If you are following meditation based approach (Rajayoga or its variants) to purify mind then you may get stuck in the experience of bliss taking it to be enlightenment which is like a golden cage. Experience of bliss is after all an experience and you are the knower of the bliss. Enlightenment is about knowing directly the knower of all experiences. Any experience comes and goes but you remain and you experience yourself 24×7. Knowing that consciousness is real you is enlightenment. You are the bliss itself and it is the mind that experiences the bliss. It is important to also understand that everybody experiences bliss every now and then. In deep sleep there is experience of bliss for everybody (that is why nobody likes getting up from deep sleep) but one does not get enlightened in deep sleep. Since an enlightened person generally has greater percentage of Sattvaguna, he/she may experience it more often. Also, an enlightened person may choose to experience bliss more frequently by abiding more often in the reflection of Self*. Bliss is a state of mind.
*Even an unenlightened person can experience bliss more frequently by abiding more often in IAmness/presence.