It sounds to me like he's writing a song about a girlfriend who is trying to take control of him creatively, so he feels. She's telling him to write about things other than the overdone topic of love and heartbreak and he's saying, "Write what you know and this is what I know". He's trying to tell her that love is the subject he knows most and feels he can write about best while she's wanting him to write on topics she deems more profound.
It sounds to me like he's writing a song about a girlfriend who is trying to take control of him creatively, so he feels. She's telling him to write about things other than the overdone topic of love and heartbreak and he's saying, "Write what you know and this is what I know". He's trying to tell her that love is the subject he knows most and feels he can write about best while she's wanting him to write on topics she deems more profound.