Vaina is a very common and versatile slang term in many Spanish-speaking countries, especially in Colombia where Diomedes Diaz is from. It literally means 'sheath' or 'pod', but colloquially it's used to refer to a 'thing', 'situation', or 'problem'.
In this song, the phrase "Que vaina tan difícil" translates to "What a difficult thing/situation." It perfectly captures the singer's frustration and the intensity of his feelings about living without his loved one, making it a memorable and culturally rich word to learn.
Diomedes Díaz opens by boasting he can survive almost anything — twenty days without food, a year broke, walking barefoot from the valley to Barranquilla — before admitting the one thing he can't stand: 'Pero vaina difícil es vivir sin ti' [But it's a hard thing to live without you]. The title phrase itself, 'vaina,' is Colombian slang for 'thing' or 'hassle,' so 'Que vaina tan difícil' lands closer to 'what a rough deal' than a literal translation. In the second half he flips the same boast toward forgiveness — he can survive gossip, envious rivals, even a friend who wrongs him and curses him back, 'Y me maldice haciéndole bien sin importarle' [And curses me while I do him good, not caring] — but never her absence, closing on the same line four times over: 'Que vaina tan difícil es vivir sin ti sin ti que dolor' [What a hard thing it is to live without you, without you, what pain].