Hi Julian.
I will try to be constructive...
Art/Lyrics/Music is a personal thing and everyone has a different opinion and so you may disagree with all or somethings I say buddy, which of course is fine, I'm just trying to help!
I think the general idea of the song is their you just have to fill in more of the details, paint a picture more about what's going on with you two, exactly how you feel at times, how she may feel, maybe some more of the history. As someone else said it's more of a letter at the moment. A song has got to let someone in and build more detail.
You should definitely take this out/re write it:
"I thought it’s okay if you write him
and it won’t be wrong for our thing"
You're concentrating way too much on making it rhyme everywhere. I think this is what is stopping you developing some of the ideas and words further. Songs generally have to rhyme in places, but you don't have to make it every line. Maybe change the rhyme scheme, e.g. A,B,A,B,C,C, or even just A, X , A, X B, X B, C, C, - x meaning the line doesn't rhyme at all.
Just an idea that may work for you. Type up the lyrics. cut each line out, keep the lines you are really happy with and muddle them up, then you need to fill in the gaps. David Bowie used this method for a couple of songs.