I need to edit the thinking behind the question before I can answer it. Your significant other doesn’t really lose interest in you first. They lose interest in the relationship first. And even that’s not really first. There’s a lot of little things that relationships are composed of that they lose interest in before they lose interest in the relationship itself. Such as sexual interest. Or working out their problems with you. Or even something as small as your weekly dates.
Once they lose interest in the relationship, there’s another long way to fall before they lost interest in you altogether. It starts with just losing interest in how you’re feeling. What your interests are. Eventually they may lose interest in even being friendly. Then they don’t even want to be civil anymore. Then they start actively avoiding contact. Then they’ll start with the hallmark cruelty of someone who really and truly just doesn’t give a shit about you anymore.
Point is, it doesn’t happen all at once. And the reason it even gets to the “lack of interest in the relationship part” to say nothing of the horrible prospect of them losing interest in you as a person is because you missed signs that you probably should have noticed years ago. People don’t want to be mean, so they pull punches and don’t say what they mean early on. They stop pulling punches and start being completely truthful when they stop caring about you.
These things are the mark of a relationship that’s happening for all the wrong reasons. If you’re in a relationship like this, and it’s gotten to that stage, then likely it’s broken beyond repair. I read an article about a study where they filmed couples counseling sessions analyzed them systematically along with the success rates of their relationships. “Open contempt” was the single biggest marker of a failing relationship.
The biggest reason why we don’t pay attention to early signs is because we don’t want to see them. No modern relationship can survive without good healthy communication. Stop ignoring signs and start communicating when you see them.