Honestly, I think the flow was COMPLETELY off in this poem. I think that you should switch it up a lot. One major problem with the poem, other than flow, and might have something to do with the flow is that you used 'she' and 'her' over and over. It was too much. Though, I do like how you used 'comeback kid' over and over, because that's needed in a way. She/her isn't.

Since you need a good beginning and a good ending those are the two I was going to show. But as long as you switch the last one to match the first one you have an amazing ending, I think. Instead I'll show another stanza.

beginning:

Comeback Kid always won,
She always had a hidden plan.
Known by everyone in her town,
She even had a 'biggest fan'.

[Comeback Kid never lost,
always had a hidden plan.
Known by everyone in that town,
Comeback kid even had a "biggest fan"]
- I know it's still off in flow but to me it sounds a lot better.

four:

So she couldn't mend her broken heart,
With the news being her final blow.
The gun was already at her temple,
When she thought about so long ago.

[ Unable to mend the broken heart,
with news being the final blow.
The gun was already at the temple,
when Comeback Kid thought about so long ago]

I like this... it's sort of sad, sort of dark, sort of mysterious and brave. It's emotionally valued and well written. I like the way you put the words in the order.
My only honest critique is cut some of the Comeback kid things... there's a few too many.
Great write.
:]