HEARTBEAT= The pulsation of the heart (pum-pum). A HEARTBEAT SONG is the song made by your heartbeat. Your heart beats so fast that you can hear its thump-thump, and that is a wonderful song, the song of love.

GONNA= (coll.) Going to.

BEEN SO LONG= It's been so long = I haven't heard that song of my heart for a very long time (= I haven't been in love for a long time).

TURN IT UP= Increase the volume or intensity of something. If you make my heartbeat song turn up, you make the sound of my heart (my heart beat) go faster and louder.

ALL NIGHT LONG= All through the night, for all the night. We can use this expression with "day" or "night" to say that something happened for all that time (all night long, all day long).

I'M SO USED TO (+ -ing)= If you are used to something, you are in the habit of doing it or you are accostumed to it. BE USED TO is always follwed by -ing (because here, TO is a preposition and "after prepositions and conjunctions we always use -ing):
- I'm used to getting up very early
- Megan is used to walking home every day

NUMB= If a part of your body feels numb, you stop feeling anything there. Normally, we talk about your leg, your had, etc. feeling numb, and that means you were in a position in which your flood stopped flowing there normally, so you feel nothing there and can't move it. If you move it, after a while blood flows restores and you start feeling "PINS AND NEEDLES" (an uncomfortable itchy feeling) before it comes back to normal. But if you say that you or your heart feels numb, you mean that you stopped feeling anything (no happiness or sorrow or love or anything).

PINS AND NEEDLES= (see above for the explanation of NUMB). Pins and needles is the feeling you get when a part of your body which was numb is coming "back to life". In this song, he/she had been feeling numb (feeling nothing) for a long time, but now they're waking up again and the first symptom of it is that itchy feeling on their tongue, like they need to express what they feel after so long without saying anything (because there was nothing to say).

ANTICIPATING= If you anticipate something, you expect it's going to happen and get ready for it.

WHAT'S TO COME= The things that are going to happen.

A LOADED GUN= A gun with bullets inside, ready to shoot (and kill). If you have your finger on a loaded gun, you are ready to shoot (and kill). If the gun is unloaded, there are no bullets inside, so if you shoot, nothing happens. Nevertheless, in this song they're not thinking of "killing", simply the idea of letting something "explode" (he means that he is about to fall in love)

RISING= Going up; increasing.

HAD A DOUBT /daʊt/= If you have a doubt, you are not sure about what to do, you don't know if you should do it or what you should do.

I COULD DO THIS FOR A LONG TIME= I like this so much that I want it to do it for a long time, I don't want to stop.

BREATHED= /bri:ðd/ To breathe is to take air in and out of your body (if you stop breathing, you die.

BRAND NEW= Totally new, completely new.

ONE MORE TIME= Again.