This will be a depressing one and (I'm confident) very probably not what the authors had in mind. I interpret "the shadow," the "dear old friend," and the "monsters in my head" as one and the same--namely, dark and paralyzing thoughts about life and the singer's place in it.
For whatever reason (a bad grade, a death in the family, a broken nail, etc), the lights go out, and the singer retreats to the shadow--her dear old friend--and, slowly and steadily, they dance with one another without letting go. The feelings of pain, anguish, suffering, and discontent are old and familiar, and, therefore, although they are acknowledged to be bad and morally undesirable "monsters in [her] head," they provide a sort of perverse comfort bred from that familiarity. They are her "dear old friend[s]," and part of her is glad to be back in a familiar and stable (although stable in the most unwholesome way) environment instead of the less usual and less familiar "light," which just went out, and, even worse, the jarring transition between light and shadow. In short, "[she's] letting go, but [she's] never felt better."
How to account for "feeling like a waterfall?" I would think the comparison would mean she's feeling emotional, in the midst of tumult, free-falling, but being jarred along the way down as well. She's moving "slow and steady" with her shadow, but, despite the comfort of being in the familiar position with those dark emotions, the loss of the light and, what's worse, how she lost it, is not far from her mind, and, I suspect, it's like a knife.
How to explain the last verses? The better part of her--the part of her that is more morally praiseworthy and striving to flourish in light rather than subsist in dank shadows--wants to LIVE, take chances, and put herself out to the world. She "knows" (read; firmly believes), however, that she'll never be ready to do that, because no amount of time will sufficiently heal her or insulate her from the fears of failing when taking those chances. Time, she believes, won't make her strong and brave, so, instead of braving life in the light where the lethal knife stroke may fall but joy may also be found, she resigns herself to living in the shadow with monsters in her mind, where there is no danger of a lethal and sudden knife stroke but no hope of the best of things, or even maybe of just what makes life worthwhile, either.
This will be a depressing one and (I'm confident) very probably not what the authors had in mind. I interpret "the shadow," the "dear old friend," and the "monsters in my head" as one and the same--namely, dark and paralyzing thoughts about life and the singer's place in it.
For whatever reason (a bad grade, a death in the family, a broken nail, etc), the lights go out, and the singer retreats to the shadow--her dear old friend--and, slowly and steadily, they dance with one another without letting go. The feelings of pain, anguish, suffering, and discontent are old and familiar, and, therefore, although they are acknowledged to be bad and morally undesirable "monsters in [her] head," they provide a sort of perverse comfort bred from that familiarity. They are her "dear old friend[s]," and part of her is glad to be back in a familiar and stable (although stable in the most unwholesome way) environment instead of the less usual and less familiar "light," which just went out, and, even worse, the jarring transition between light and shadow. In short, "[she's] letting go, but [she's] never felt better."
How to account for "feeling like a waterfall?" I would think the comparison would mean she's feeling emotional, in the midst of tumult, free-falling, but being jarred along the way down as well. She's moving "slow and steady" with her shadow, but, despite the comfort of being in the familiar position with those dark emotions, the loss of the light and, what's worse, how she lost it, is not far from her mind, and, I suspect, it's like a knife.
How to explain the last verses? The better part of her--the part of her that is more morally praiseworthy and striving to flourish in light rather than subsist in dank shadows--wants to LIVE, take chances, and put herself out to the world. She "knows" (read; firmly believes), however, that she'll never be ready to do that, because no amount of time will sufficiently heal her or insulate her from the fears of failing when taking those chances. Time, she believes, won't make her strong and brave, so, instead of braving life in the light where the lethal knife stroke may fall but joy may also be found, she resigns herself to living in the shadow with monsters in her mind, where there is no danger of a lethal and sudden knife stroke but no hope of the best of things, or even maybe of just what makes life worthwhile, either.