Lewis Capaldi - Someone You Loved Lyrics Meaning

So, he speaks about going under and fearing that there's no one to save him from possibly being sad and depressed. The person probably said it is either all or nothing and he feels that if he loses that person he may lose himself as well. Later in the song in the chorus he says "I was kinda getting used to being someone you loved" so this indicates that the person is probably done with with him and has had enough and later he says how the person pulled the rug when he wasn't just getting comfortable so this probably means that he usually doesn't trust a lot of people and when he does they usually pull the rug and let him fall and sort of creep up on on him like a predator. This song is about someone who is very hurt and they feel as if they don't or can't have that thing whether (love, drugs, etc) they will die or be depressed forever and they will never find another person who "numbs" all the pain.