The speaker is worn out by constant criticism and wishes to become a shadow, something that can slip out of sight and stay safe from hurt. They feel stuck in a situation they describe as "a lot of junk," and they don't believe the nagging will ever end. What stings most is being misjudged: others assume the speaker is selfish and deserves the scolding.
The shadow is a clever choice of image. A shadow is always present but rarely noticed, and only someone who really looks for it in the dark will find it. That seems to be what the speaker wants: to be left alone, but also to be sought out by someone who cares enough to search. The last line, "Thinking I'm not around but I am," carries both hurt and quiet defiance. The mood is tired, lonely and a little bitter, with a small hope underneath that someone might truly see the speaker.