My worst D&D session

This past Saturday I sat through a session of D&D that was the least fun I've ever had. It was fine the first half of the four hour session, but the second half was dismally unfun and my entire drive home afterwards I was near tears because of how unfun it was. We're playing a pre-made campaign, The Wild Beyond the Witchlight, I'm having a lot of fun with my character (an Echo Knight Fighter), and the DM is great at what he does. But as I've been thinking about why I had such a bad reaction to the second half of the session, I've been realizing some things about myself and about the campaign. First, the campaign feels more like an open world sandbox rather than a specific story we're supposed to be following. The motivation the three of us player characters should have doesn't feel strong enough to get us to actually do what we're doing, so instead (to me) it feels like we're just off having an adventure with no real driving force. Just a sort of "let's explore and help other people"…