ego

One of the main reasons people start working to "dissolve the ego" is to disengage from dysfunctional and reactive patterns developed in childhood.

In the space that emerges as it's grip is loosened, a new ego can then be created and deployed to replace it. Most come to identify with this new ego but we don't need to be it, we can just use it.

In the act of asserting a boundary for example, ego is used to repel an outside force from our psychic space by providing a clear sense of self and not-self, of your beliefs/needs/desires and mine. Without this clarity, we're vulnerable to manipulation. If we're aware of our boundaries, ego can be deployed to assert our own preferences. We can then return to the reality of egolessness, since it's really only by pattern and repetition that the illusion of a stable egoic identity is maintained.

With those in abusive relationships or cults, the dominant party diminishes the ego of the submissive to the point that their needs and desires can be overriden by the wills of others. They become easily controllable because they lack the awareness, boundaries and ego strength to assert personal will and autonomy.

This isn't to say that it's always harmful to relinquish control. All balanced relating involves surrender to some extent, and in some cases doing so more fully can be beneficial or enjoyable. The potential for abuse is there though, in which case awareness and the capacity to self assert remains valuable.