Long relationships are not supposed to feel like the first three months forever — that's not a design flaw, it's just what long-term attachment looks like once novelty wears off. A relationship rut, where things feel routine and a little flat, is normal and usually temporary. Falling out of love is something else: a gradual, more permanent withdrawal of investment.
One useful distinction is whether you still want things to get better. In a rut, you're bored or tired, but you still care about fixing it — you'd be relieved by a good date night or an honest conversation. When feelings have actually faded, the idea of “fixing it” feels exhausting rather than hopeful, because some part of you has already stopped believing it's worth the effort.
Another marker is how you talk about your partner to other people, or to yourself. In a rut, the complaints are usually situational — “we haven't had a real conversation in weeks.” When love has faded, the complaints get more character-based and permanent-sounding — not “we're stuck,” but “I don't think I like who they are anymore.”
If you're not sure which one you're in, that uncertainty is itself useful information: it usually means you're still invested enough to want an answer, which is a reasonable basis for trying — a real conversation, a change in routine, maybe outside support — before assuming the worst.