[I always said that I'd mess up eventually
I told you that, so what did you expect from me?
It shouldn't come as no surprise anymore
I know you said that you'd give me another chance
But you and I knew the truth of it in advance
That mentally, you were already out the door.]

The first verse refers to Harry, and the management overall. Modest! made Louis hide his sexuality from the world, trying to keep the image of a perfect 'straight' boy band, because they wanted their fanbase (which is over 80% female) to think they had a chance with at least one of the boys. In mid 2013 Larry stopped being as close as they used to be, and many of the fans suspect that they were in a relationship, causing the management to force them apart. In my opinion, I think that Modest! thought that Larry were giving too many hints, and the eagle-eyed fans caught right onto it. I think this is what Louis means by 'messing up'.
Larry were probably given a few chances in the beginning, which I think is the meaning of the second verse. It can also refer to Harry, idk, believe what you will believe.
The last verse, and the ending of the stanza, refers to the fact that this was very emotionally and mentally harming to both Harry and Louis.

[Never thought that giving up would be this hard
God, I'm missing you and your addictive heart.]

Sorry, but this just screams Harry Styles to me.

[You're the habit that I can't break
You're the feeling I can't put down
You're the shiver that I can't shake
You're the habit that I can't break
You're the high that I need right now
You're the habit that I can't break.]

This stanza can refer to One Direction as a whole, but sorry it still screams Larry to me.

[I took some time 'cause I've ran out of energy
Of playing someone I heard I'm supposed to be
But honestly, I don't have to choose anymore.]

This is honestly the most heartbreaking stanza to me. It again implies to the fact that Louis is tired of being someone he isn't. Louis is perfect the way he is, and about being gay, it's the freaking 21st century, people! No one cares about your sexuality anymore! You can be whoever the f*ck you want and love whoever the f*ck you want! Louis is right that he doesn't have to choose anymore, and I'm honestly so proud of him.

[And it's been ages, different stages
Come so far from Princess Park
I'll always need ya
In front of me, in front of me.]

This is the most obvious Larry reference in the entire song. 'Princess Park' refers to when Larry lived together.

[You gave me the time and the space
I was out of control, and I'm sorry I let you down.
Guess that I know what I already knew
I was better with you and I miss you now.]

This in my opinion, refers to Larry's relationship. This song shows how much Louis has grown in the past few years