There is a lot in this short poem and I think it's power lies in its brevity. You manage to pack a lot of emotion and feeling in and it certainly managed to keep the reader interested throughout.

'Twas plain to see in the light
She was not plain at all'

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I think this is quite a gripping opening line: it is simple but effective. It's not over the top; stating about her tremendous beauty or describe details like how her eyes glistened etc. By saying simply how she wasn't 'plain at all' the reader knows, in a more powerful, subtle and certainly sincere way that the author has admiration for her and is attracted to her. There is a sense that maybe other people might describe her as plain, as the speaker seems the need to clarify or respond to people who think that, and this is very well done also. The use of 'at all' is a nice touch. the subtle repetition of 'plain' and the wordplay with it is clever and effortless too, and it flows well. The one thing I didn't like about the opening was that you use 'twas instead of 'it was' - I generally prefer simple language such as 'it was' rather than somewhat antiquated language such as 'twas, which stands out a little from the rest of the poem.

I like the repitition of glass again: it's simple but effortless. There is the sense that the speaker is being honest and sincere in this poem without trying to romantise it too much: almost stating it as fact. The rhyme works well until this line:

'Get close to her light soul so fair'
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The 'so fair' bit here I felt was only put in to fit the rhyming scheme and it wasn't seemless like the rest of the rhyme. It disrupted the flow a little for me as it seemed somewhat forced. Also, 'light soul' is one of the weaker images in the poem in my opinion: I am rarely a fan of the word 'soul' in poems anyway, it is one of my poetic pet peeves!

'In the shadow of my mistakes
She would be with me everywhere'
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I really liked these lines and felt they had an impact worthy of ending a poem. Again, consistant with the rest of the poem, this line is simple on one level but revealing of some of the speakers deepest emotions at the same time. It really adds a melancholic tone. In fact, the last two lines are a let down of sorts after that line, as they fail to top it and go back to a slightly more forced ryhme. Also, I wasn't a big fan of the line 'I must have committed a mortal sin,' because I think the man should know what he has done but cannot avoid/change it, and this would add to the tragedy of the poem and increase the readers sympathy for the speaker. We don't need to know what it is he did or where it went wrong, but I think we should know that he knows what he did or where it went wrong, and we can only guess at it.

So, all in all, a poem of real depth and impact, despite one or two little things mentioned above and the fact that I would probably leave out the last two lines altogether to maximise the potential of the 3rd and 4th last lines which provide a more natural ending, or look again at the ending. But good work