That's just an issue with the game, the party never really feels like it comes together and feels like a cohesive group. It just always feels like Tav and our friends, with us being the ony significant relationship the others build within the group. Shadowheart and Lae'zel have more of a meaningful arc, but it never feels like, beyond them, the rest of the party graduate from the "getting to know each other" stage.
As for Mystra herself, I don't see the relationship as actively abusive, but I do think it was ultimately unhealthy for Gale for a number of reasons, high among them being the simple fact that she's a goddess. The power balance of that alone makes any relationship kind of dubious. But beyond just the power balance, Mystra's just endowed with so much responsibility that any relationship is almost doomed to fail. She may love mortals, but she is in a position to impact basically all of them with her choices. She can't devote herself to Gale because she has to see to the well-being of every mortal. That's only further complicated by the fact she's the goddess of magic, which Gale already loves deeply and ties to his identity. Of course he's going to come away from that relationship feeling lesser. Fundamentally she could never put him first when it really mattered. Even if she would have truly wanted to.
