boyfriend wrote a song about his ex

hi so my (25m) bf and me (22f) have been dating just shy of a month, so I know anything could happen and I'm trying not to overthink or self sabotage. he makes music which i want to support fully. the first day we met he was filming a music video, well it came out today and he's been promoting it all week.. it's about his ex/babymom and pretty much talks about how he wishes things were different, and wants to run back to her and overall makes me feel very insecure with the relationship. he did write it before we ever met.. but there was an issue that came up about him trying to rekindle things with her 2 weeks before we got together. he tells me to not take it personally and that he definitely doesn't have feelings for her anymore. but, I can't help the depressing feeling in my heart hearing these lyrics knowing it's about her. I really care about him, but I can't stop feeling like he might only be with me because he can't be with her. other than this song he doesn't make me feel that way, which is why I don't want to say anything but also I can just feel myself being sad today knowing that my boyfriend just released a song about him essentially wanting his ex back that he was with for years.