Yanno, I think my favourite part was how you said thank you at the end. It showed how greatful you were for being shown the ugly side of a situation. It seems like an opposite to your other poem without actually being an opposite. (If that doesn't make sense dont bother haha.) Anyway, I really like that you were able to do this. You took the love and made it so unsure, so unfaithful even. As many readers will be able to connect with this one as with your other one.

The words you used weren't particularly strong, but sometimes when the vocabulary is toned down a little it's better for the readers anyway . (Of course that depends on the kind of readers you are getting. Like comedy depends on the happiness and number of people in the audience and such.) So, I am going to mention that you could read some others things and maybe do something to challenge your vocabulary; however, don't take it too personally or close to heart, as there are plenty people out there who prefer it simple anyway. :)
Your rhythm is good, but I'm curious.. Have you ever written anything without rhyming? Straight thoughts, turned into an unsure broken up write of poetry?
If you ever do, or if you already have, I'd love to read it :).
Hmm.. You have a bit of a gramatical error in this stanza, but you can probably find it yourself so:

You were so sweet,
Now you have turned sour,
You now like lightning,
Cutting off my power.

Thats just from a typo or something I'm guessing. They're easy to look past.

I would kind of wish to see this stanza follow it's own meaning a bit more:

You are the sun,
You burn like fire,
You have cut up my heart,
With barb wire.

When I say that, I mean that it goes from talking about a heating sun, to barbed wire. Maybe we could make it something more like:

You are the sun,
My ball of fire,
Burning my heart,
Is your only desire.

... Something along the lines of that anyway. I'm sure you could do a better job. If you don't agree with any of this, feel free to brush it off of course :).
Thanks for entering the contest. I think I may be leaving PFP though, as we seem to be more and more inactive every day.
Have a wonderful everything and keep writing. :)