Longing in a Stable Marriage

I’ve been married to my partner for 20 years now. We have two kids and our life is pretty stable, financially comfortable, but not extravagant. Everything seems fine on the surface, yet I can’t shake off this nagging feeling of emptiness inside me. It started small, with fantasies about someone else, but as time went on, these thoughts grew more vivid and frequent. They no longer just occur randomly; sometimes they consume me, especially late at night when my mind wanders and I feel so disconnected from my partner that it feels like there’s an ocean between us. I’ve always been honest with him about our sex life, we talk openly but never push each other beyond what makes both of us comfortable. Yet, here I am, fantasizing more than ever before, craving something or someone that I can’t quite define or grasp hold of. Is it possible to feel this way and still be in a good relationship? Am I the only one who feels like they’re missing out on something even though everything is great on paper? Do you think talking to a therapist might help me understand what’s going on inside my head?