Finally, found it. The mixtape yo yo guys.
Thanks for @white_tofu for the lyric, 'cause ya know, I didn't really like to write down something from a vid :3

Romaji
Geoulsok bichwojin
naega eosaekhae jiteojin
dareun bunwigi eoneusae
meorisseok kkwotdeon kkum
irwo jarago isseo

Silgamhae geugan byeonhwareul
mot ijeo geu kkamadeuk
haettdeon Seoul City cheot insang
hangyeoulboda sirin nareul bonae
geudonganui byeonhwareul changjakhae

Nothing is easy
galsurok kkaedara
(yeol ahop sal dino)
Geureohkie onereul neukkyeo
Oh yeah yeah yeah
Maeil maettdeon chekeu necktie
Ije pure saeropge joyeo mae
Geudonganeul baldodumhae dasi sijakhae zero

Hold up gyobokdo ttae mudfi
jeon kkumeul chajaga
Bakkwin natbame ttameuro
tteugeopge nal salma
Nalmada hancheung hancheung deo ollaga
Nae haknyeongwa gati sok dareun chaek gabang
Jeomjeom geureohke chingudeul
bodan mudaega nae ilbubun
Dwaetgo mitgiji anheul geot gatdeo  oneul
Eojeui chukha tto naeireul allyeo

Nothing is easy
galsurok kkaedara
(yeol ahop sal dino)
Geureohkie onereul neukkyeo
Oh yeah yeah yeah
Maeil maettdeon chekeu necktie
Ije pure saeropge joyeo mae
Geudonganeul baldodumhae dasi sijakhae zero

Jorephae hyungdeul dwittara beotne yeah ah yeah
Heona yamanfui hyeonsilhwaneun cheotjjaeboda
apjarie nine to zero
Suneunggwan dalli noryeogeun jaesueopji
Tteotji tteotji ije kkocheul piwo nine to zero
Suneunggwan dalli noryeogeun jaesueopji
Tteotji tteotji ije kkocheul piwo nine to zero

Geoul sok bichwojin naega eosaekhaji anhgi
Anhgi yeah

English
The me that's reflecting in the mirror seems unfamiliar
A deepened, new aura the dream that I dreamed of in my mind is coming true and growing.
I feel it, the change during then, I wont forget it.
The first impression of Seoul City that seemed distant
Spending a day colder than the middle of winter, I write about the change since then
Nothing is easy
I learn this the more time passes
(19 year old Dino)
Because of that, I feel today oh ye ye ye
The checkered necktie I wore everyday
I take it off now and tie on a new one
I stand on my tip-toes and start again, 0
Hold up, I look before my dreams even before my school uniform could get dirty.
And with my day and night flipped, I lived daily, heated with sweat
Each day, I go up one more flight
My backpack that had different things inside from people in my grade
Like that, gradually, the stage becomes larger part of me than friends
And the congratulations I got today and yesterday that seem unbelievable, tell me about tomorrow
Nothing is easy
I learn this the more time passes
(19 year old Dino)
Because of that, I feel today oh ye ye ye
The checkered necktie I wore everyday
I take it off now and tie on a new one
I stand on my tip-toes and start again, 0
I graduate and follow the hyungs ye ah ye
but the reality of my ambitionis changing the font number from 9 to 0
Different from SATs, effort doesn't work
Now blooming flowers 9 to 0
Different from SATs, effort doesn't work
Now blooming flowers 9 to 0
So that I wont be awkward with the self reflected in the mirror ye

English translation by @eostower on Twitter