ARENT’ YOU SOMETHING…= (coll.) It is very common to make an exclamation using a negative sentence.
- Aren’t you beautiful! = You are very beautiful!
- Isn’t this a wonderful day! = This is a wonderful day!

CAUSE= (coll.) Because.

MIRROR= A surface that reflects an image (see picture)

I CAN’T HELP BUT [+ infinitive without to]= If you can’t help but do something, you simply can’t stop yourself from doing it. You feel like an irresistible impulse to do it.
- When I’m with you, I can’t help but feel so happy all the time.
- Every time she is near, I can’t help but feel angry.

NOTICE= If you notice something, you get that information through your senses.
- I noticed someone was in the house because I saw a light in the bedroom window.
- She noticed me when I touched her back.
If the information doesn’t get to you through your physical senses but through a mental process, then you can’t use NOTICE, for that we use REALIZE:
- I realized she was very tired when she didn’t like to dance to her favourite song
- They could never realize how much they were hurting me every time they offered money to help me.

GLARE= An intense, blinding light.
If you look in a mirror but there is a very strong light (such as the sun) behind you, you can’t see the image because the glare of that light is blinding you.

HARD= Difficult.

SOUL= Spirit, the non-physical part of a person.

THERE’S NO PLACE WE COULDN’T GO= This sentence may be a little bit confusing because it uses two negatives. In English, two negatives cancel and produce a positive statement, so this sentence means: We can go everywhere.

PULL YOU THROUGH= Since they’re talking about a mirror, made of glass, if I’m on this side of the mirror and you are on the other side of the mirror, I can reach out, take your hand and pull you through the mirror to bring you to this side, with me.

GOTTA= (coll.) Have to, have got to, must.

WANNA= (coll.) Want to.

VACANCY= A vacancy is an empty space. In this case it is the void (emptiness) he feels in his heart because he feels alone and needs someone to love and fill that void inside of him.
If we are talking about hotels, vacancies are empty rooms that you can use. When a hotel is complete and can’t accept more customers they hang out the sign “No Vacancies”.

HOLD= Have in your hand; possess.

FIGURED IT OUT= If you figure something out, you solve that problem. In this case it probably means that he finally was able to understand how much she meant for him, he finally realized she was like his reflection on the mirror, the person who could perfectly match him, so when he realized that, it was easy going back to her and telling her all the things he’s telling her in this song.

RIGHT HERE= We can use RIGHT to emphasize time and space:
- She lives right there, next to the bank.
- We’re leaving right today so I’m in a hurry.

ALL ALONG= All the time, always.

LIKE= (conjunction) as if.

STARING= To look directly and fixedly.

I COULDN’T GET ANY BIGGER WITH ANYONE ELSE BESIDE OF ME= When I’m with you I feel very big (good, important, fulfilled, strong). If I were with any other girl, I wouldn’t feel so big as I feel when I’m with you.
ANYONE ELSE= Any other person; someone different.
BESIDE OF ME= By my side, next to me. (in BrE we would say "beside me", without "of")

CLEAR= Obvious

AREN’T YOU SOMETHING!= (coll.) Again we find a negative exclamation, which has a positive meaning. This means: You are very important! You are great!

AN ORIGINAL= Someone special, unique, original, with personality.

MERELY= Simply, just.

ASSEMBLED= Something assembled is manufactured by joining (assembling) pre-made pieces together, so they are produced in series and they are all the same.

WHITHOUT YOU / ABOUT YOU= Notice in these two cases the assimilation rule number one:
T + Y = CH
(You can find more cases of this kind of assimilation in other parts of the song)

BACK= This verb particle is often used to express the idea that the action goes from A to B and then returns to A, so when they say “I can see you looking back at me”, it means that I looked at you (from A to B) and then you looked to me in response (from B to A), so I started the action and then I got the same action back:
- She phoned me three times this morning, but I’m not going to call her back (= return her call)
- That’s the book I gave you last month, give it back! (I gave you the book, now I want you to give it to me)

WHAT’S UP= (coll. esp. AmE) Hello.

DOUBT= /daʊt/ Hesitation. NO DOUBT= It's obvious; I'm totally sure about it, I know for certain.