What a very touching poem, I think it shows the true despair of a mind that is filled with darkness. Like all the hope for life has been drained.
There are a few great metaphors used in the poem, all working really well. The description of the sky, and how she was a star in it, and then the fallen angel, it was worded so beautifully. I also like how you end the poem by bringing the sky back like it was in the beginning, where she feels free and accepted, at peace with herself.
I think you chose the animal well as a snake, beneath her skin, squeezing life out, as this is exactly what snakes do and is so easy to picture this scene. This was done well.
I also like the following stanza where you say time promised nothing, such a true true statement!