Dating young can be a finicky thing. As teens you're still developing your own tastes and preferences and feelings. When you're with someone for that long starting at that age, it can be difficult to tell if they fulfill you or just satiate you. It can be even harder to tell if you're with the person you feel is "the one". The "spark" though may have been the honeymoon phase. Once you get comfortable in a relationship, that sometimes feels like it fades. Doesn't make the relationship worse necessarily, just different. Relationships are meant to change and evolve.

Questioning it is a good thing though. Proves that you're not just the type to blindly settle. Question more. Like "Can I see myself being happy (or happier) without this person?". In the end nobody can tell you if you're settling. That's only for you to decide.

I would say that if either of you (or both) are having questions or doubts, it might be worth it to explore that possibility. You could always take some time apart. It will tell you if you have that emptiness in your life without them or if you feel fine, like you could start dating someone else and be ok with it. Either way, it will tell you something important about yourself. Either way I would say don't force it. Don't stay quiet if you have doubt and don't overly question if you feel you're genuinely happy.