The "midst" of something in the Bible represents the thing that is most central and most important to the spiritual state being described, the motivation that drives everything else. In general this will be something we love or feel, because at the core of things we are what we love; our loves define us.

(References: Apocalypse Revealed 44, 90, 268; Arcana Coelestia 2252, 2940, 10153, 10365, 10557, 10635, 10641)

2879. The Lord enters in with good by way of the inmost part of a person and there joins it to truth, for it is in that inmost part that these must take root. Unless a person is interiorly in freedom as regards all his affections and all his thoughts he can never be so organized that good and truth strike any root.