Here’s the link to the song:
いつもどおりのある日の事 - On the certain day that was the same as usual, that thing
いつもどおり - as always; as usual
ある日 - that day
事 - thing; matter
君は突然立ち上がり言った - You suddenly "started to say"/ "stood up and said"
君ーyou
突然-suddenly
立ち上がり-stood up/started up/took action
言った-said
立ち上がり言ったー same as 君は立ち上がって言った (verb stem can be used as a formal version of te-form)
「今夜星を見に行こう」- Let's go look at the stars tonight
今夜ーtonight
星ーstar(s)
見に行く-to go to see
こう ending - makes it more of a “let’s/why don’t we” nuance
「たまには良い事言うんだね」- You say good things some times, だね (kind of teasing them by suggesting that they usually don't...)
たまに-occasionally
良い事ーgood things
言うんーyou say + an ん for …texture? not sure
だねーdon’t you, (or something like that)
なんてみんなして言って笑った - (...that sort of thing,) everybody used to say and laugh
なんて - making light/jest of the previous quote
not sure what this して is doing here
明かりも無い道を - walking down a lightless street
歩いた - walking, from next line
バカみたいにはしゃいで歩いた - prancing about looking like fools
はしゃぐ - to prance/frolic
抱え込んだ孤独や不安に - avoiding that loneliness and anxiety we bore
抱え込む(かかえこむ) - to bear, carry, be saddled with
(added “avoiding” for good flow with the next line)
押しつぶされないように - so as not to be crushed thereby
TN: not sure who’s the subject of these lines, we? みんな? 君?
真っ暗な世界から見上げた - looking up from a pitch-black world,
夜空は星が降るようで - it seemed the stars themselves would fall from the night sky!
いつからだろ 君の事を、追いかける私がいた - there I was, always following you along (metaphorically); how long has it been now?
いつから - since when
君のこと - indicates metaphorical nuance of following a person
TN: combined 2 lines, bit of poetic license
どうかお願い - Please, somehow,
驚かないで聞いてよ - listen and don't be surprised
私のこの想いを - to these thoughts of mine!
「あれがデネブ、アルタイル、ベガ」 - "There's Deneb, Altair, and Vega"
君は指さす夏の大三角 - You said, pointing toward the Summer Triangle
覚えて空を見る - remembering that, I look at the sky
やっと見つけた織姫様 - At last I found Vega
やっと at last, at length, barely, narrowly, just
見つける to find; conjugated to past tense
織姫（おりひめ）Vega
様（さま）formal name ender (for a person)
だけどどこだろう彦星様 - But where could Altair be?
だけど however
どこ where
だろう plain form of でしょう (from です), used to indicate conjecture
彦星（ひこぼし）Altair
様（さま）formal name ender (for a person)
これじゃひとりぼっち - all alone here
これ this（これで here)
じゃ combination of で and は particles
ひとりぼっち aloneness, loneliness, solitude
*I’m not sure who is alone here - is it the singer, identifying with Vega; or Altair, who can’t be seen?
probably the former – bblum
楽しげなひとつ隣の君 - with you seeming to have fun by my side,
楽しげな = 楽しそうにしているさま
this construction seems to be expressing spectulation about 君’s emotional state
私は何も言えなくて - and me not being able to say anything
本当はずっと君の事を - the truth is since always
どこかでわかっていた - somewhere, i understood you
not sure about this, wouldn’t it make more sense in context as “you understood me?”
を isn’t the right particle to use with 分かる anyway 
見つかったって - that "i found you!",
届きはしない - it never reached...
reached me? or reached you?
だめだよ 泣かないで - "that's no good, don't cry",
そう言い聞かせた - you said to me.
強がる私は臆病で - pretending to be strong, but i was scared
臆病（おくびょう）- cowardice
興味がないようなふりをしてた - i pretended like i wasn't interested,
だけど - but...
胸を刺す痛みは増してく - this pain piercing my chest keeps growing
してく = して+行く
ああそうか 好きになるって - of course... "i'm falling in love"
こういう事なんだね - this is that sorta thing, isn't it?
どうしたい?言ってごらん - try saying, "what do you wanna do?"
ごらん - like ください with more “have a try and see what happens” nuance
i’m not sure how this is supposed to flow into the next lines though
心の声がする - the heart's voice calls out
〜がする - to be expressed, usually of a physical trait
君の隣がいい - i'd love to be by your side
真実は残酷だ - but reality is cruel...
言わなかった - i didn't say it
言えなかった - i couldn't say it
二度と戻れない - and now i can never go back
あの夏の日 - That summer day
あの - that (which is far from us distance-wise)
夏 - summer
日 - day
きらめく星 - those shining stars
きらめく - to sparkle, shine
星 - star
きらめく is ‘qualifying’ 星 to say that the action of shining is what the stars are. It’s like a verb adjective
今でも思い出せるよ - Even now I can remember
今でも - even now
思い出せる - potential form of 思い出す - to remember - can remember
笑った顔も - your face when you laughed
笑った - past tense of 笑う - to laugh
顔 - face
も - used to make a list of 'also’s
笑った is ‘qualifying’ 顔 to say lit: the face has laughed
怒った顔も - your face when you got angry
怒った顔 - angry face
も - used to make a list of 'also’s
You could also split it into 怒る(to get mad) and 顔(face), so literally: “face has got angry”
大好きでした - I loved it
大好きでした - past tense of 大好き - love
おかしいよね - isn't that crazy?
わかってたのに - even though you understood me
pretty sure 君 makes more sense as the implied subject here
君の知らない私だけの秘密 - this secret of mine alone, that you never knew
夜を越えて - crossing across the night,
遠い思い出の君が - the you in this faraway thought of mine
指を差す - pointing up at the sky
sky is implied of course
無邪気な声で - with that innocent voice of yours
Sorry if the formatting for this is bad, if you have any suggestions feel free to say so.