Another term for psychologist and psychiatrist is the noun shrink. Stop shrinking me means, don't psychoanalyse me.
Reality, notice the word Real-, refers to the real/true world, as opposed to a world based in fantasy. You need a serious reality check means, you need to see the world the way it really is; stop living in a world of fantasy.
Cut . . . slack refers to rope. If a rope is too taut, too tight, then it's not slack. To make the rope slack, you have to loosen it. The idiom cut somebody some slack means to loosen contraints.
I am stressing is short for, I am stressing out about something.
There isn't an a or a the before great: with great authority on the subject.