I have a couple of suggestions.
Firstly, I think it would read better if regularized to three lines of similar meter, a bit like 시조(a Korean traditional poetry genre which uses three balanced lines).
Secondly, in a poem, 탄환(bullet) might sound better than 탄알 which is too much of a street word.
With these ideas, I tried rewriting it like this:
성주 내 마음을 녹여주는 이
탄환 돼 내 가슴에 사랑 쏘고
절망은 그대 슬플 때 내 마음
In English the equivalent may be:
Sungjoo, the one that warms my heart
Bullet of love you shoot my heart with
Despair is what I feel when you're sad