as for 괜히, its meaning is much more informal and not as strong as “in vain.” It should mean more like “without need/purpose” i.e. “damn it, i 괜히 took the SAT. (b/c) I got a really high score on the ACT.”

i havent been to korea in 9 yrs, so I’m not 100% reliable.

let me try to translate:

I try hard to not to be interested (in you) b/c my interest (맘 – thought, mind, intent) in u might be revealed, (or) be openly (훤히) obvious.
Words that I pointlessly throw on as if I’m simply playing around…

I don’t see you as a woman (perhaps means his interest/love in “you” is deeper than one that is merely sexual/physical attraction, or might mean “I’m invisible to you being a woman”)

Looking at you from the side for a long time…
Would she know me pretending to do other things when our eyes meet…

It’s really a big problem (for me). I keep wanting to see (you)…
I can’t do anything (lit. “work”) b/c I keep thinking of you… (는데 has effect of “although/even though” or “btw” … in this case, it’s more of “by the way”)
It’s a really big problem. If it goes on like this I might die (in English, “I might die” is understood with much more level of certainty & magnitude of fear; in Korean, “죽을 것 같은데” carries a bit more sense of sarcasm & speculation)
btw (my) chest (i.e. heart/soul) hurts very much
Why do you make me fall in love with you, expecting me to do what (like “what do you expect me to do?” in a hopeless sense) & you planning to do whatever

Perhaps because you will start avoiding me, perhaps because my presence might be too burdensome (부담 = burden) (to you), approaching you becomes (a) fearful (thing).

What if (lit. what should I do if) seeing each other often or standing next to each other becomes very awkward

Rather I will look at you for a long time alone
since we wont need to do a sad ending to our relationship (lit. separation)

(repeat)

at least couldn’t a miracle happen and make you love me?

It’s really fine. (means more like “I’m so glad”) Because I loved you first, because only I miss you, it’s a really hard situation/relationship (lit. work)
It’s really fine. Because I don’t have to get hurt
Because you won’t need to cry
Yeah, you keep smiling as you do
Only I need to be hurt.

It sounds much less dramatic when translated in English. It’s a good lyric, I must say.