Our Last Braves Game

[busker]

Why would you need to know “what ruin?” and “what crack?” to understand this? - because otherwise they're filler words. Filler words cheat the reader.

I’m sure you do understand it, but you just don’t know what specific ruin or what specific crack… because I haven’t put those bits in. I leave it up to you, dear reader, to imagine the ruin and imagine the crack and imagine what those words might mean in relation to the theme of the poem—they may even mean more than one thing at a time! - no, the reader is not sufficiently invested in the poem in L1 and L2 to make the mental effort. Because these are filler words

As for the second, in terms of relationships, I’m not sure it is an expected two-way loss. Often, someone gets dumped, and they are the loser. In fact, “it’s their loss” is a common refrain of the friend or relative of the dumped. 
it comes across as a lazy continuation of 'I lost my girl', but that's ok

Because to lose something implies you cared to have it. You seldom really lose what you throw away. So, I thought I would balance the equation like that to show the insecurity of the author. 
None of that 'balancing' comes across in the poem.

Also, it implies she's not dead (this isn't a dead girlfriend poem)... and, lastly, I think it’s funny humorous word play.
it most assuredly is not word play

And no, there is no contextual joke in the title. 
my bad, I assumed there was cleverness.

In fact, it's about as straight forward a poem as a poem can be, so I'm a little surprised you're struggling with it. 
'struggling' overestimates the merits of the poem. I'd say that it's a piece of doggerel and I was expecting something a little cleverer.

You're funny. Why were you expecting something clever? Or even a little clever? Anyway, you have one thing right, it probably isn't technically "word play". I meant I was being playful with the phrasing. Like that Beatles song "I once had a girl or should I say she once had me". 

I understand we could go back and forth with this forever... but, I'm not getting your reasoning for any of this, frankly, vitriolic criticism. But sticking to the first line, I have used 2 nouns as a setting for the second line. How is that filler? "Outside a ruin, beyond a crack, I lost my girl...". In a 4 line poem (with an semi-explanatory title) you don't really need too much investment to assume these words are scene setting as well as metaphorical. Now, don't get me wrong, this clearly isn't meant to be some epic poetry. But, doggerel? Really? 


*note—I just noticed you said initially "can't understand the first two lines, either..." implying you didn't understand the title. Implying you had read the poem and tried to find something "clever" in the title based on south and north Decatur etc. Isn't that how we read poetry? If the first line of a poem is a little vague and non-specific, we don't just write it off and say "I have read this first line but it is too vague, therefore I will dismiss it as filler and then read the rest of the poem and never refer back to it again."

It is filler because it stands in for rhyme and meter. It pads out the line. It is a tired old trick that should be avoided.
Hope that clarifies.