Exactly. In my headcanon, Terrwyn is slowly letting go of the Dalish, and the vallaslin was just the final step. As a First, I assume she had a Keeper's ring to show her role among the Dalish. She has it on her finger at first, a cuff binding her to her duty. Then, she hangs it around her neck - chaining herself to her people because she is afraid to lose them as well as herself; it could also represent a noose, because she was never given any freedom among her people, and if she doesn't allow herself to let go she won't actually know herself. Then, she utterly throws the ring away. That ring binds her to a culture that is not saving itself as they believe, and she just doesn't believe in it anymore after everything she's seen.
Removing the vallaslin was frightening because when she looks in a mirror she will finally see that she is no longer Dalish. But she felt it was necessary not just to move on, but to gain closure with her feelings of her people as well as the loss of her clan. She no longer saw herself as one of the "noble wanderers". Terrwyn thought Solas would be there to help her come to terms with everything together, as they always did. Instead, she was left with a bare face, and her emotions in turmoil.
I think some independence from Solas will give Terrwyn time to come to terms with what she wants and who she is.
*snip*
I can totally relate to the agonies of letting go of something as precious as the last living remnants of your entire people (as she saw it). The bitterness for my Lavellan was the result of really, really wanting to continue to believe that what her people were doing was right and good at the heart of it, but she could not continue to deny that they basically got everything wrong. Abelas was the second to last nail in the coffin (I've seen this sentiment elsewhere), the fact that an actual ancient elf would consider her a shem and then Solas, as we know.
Interesting thought: I've mentioned the wisdom/pride spirit thing that clings to Solas, his fascination, possibly part of his being (or wholly his being), his constant references to it, his mental control in the Fade that keeps demons from being demons around him; all of this points to what is quintessentially Solas, a seeker of wisdom and a victim of his own pride (possibly), but either way ultimately he is pitted against the arrogance he sees in modern elves. It is that quality in him, of moving on the wisdom-pride axis, that enables him to recognize his own bias and let it go if you call him out on it. So, considering a person he deeply cares for, his gift is that of wisdom (and more importantly: humility) in the face of false pride. Should Lavellan continue to be prideful in ignorance? Whether it's an innocent pride that gives her strength or an overbearing arrogance, the point is, while removing the vallaslin is a symbol of action, it is the knowledge of what it truly is that is the gift itself. Note that (if I know correctly) Solas does not disapprove of your choice to keep it if you do decide that. It is up to Lavellan to carry the burden of this knowledge or not.
Therefore, this entire question becomes less about Lavellan's ties to the Dalish and more about how she chooses to conduct herself as a Dalish - or more importantly as an elf - after gaining this sliver of knowledge. I understand this clinging that I saw expressed in my Lavellan's inability to face these difficult junctures without tears inside, and her constant attempt to defend what she is and where she came from, to the elves of the Dalish clans the ancient times are idolized, revered and respected. It's like finding out your best friend used to drown kittens. (Blackwall, anyone?) To my Lavellan being Dalish was never a shackle, but something very fragile and wondrous - she never realizes how much strength she draws from her people's legacy, from her heritage, the roots that distinguish her from others until all of that is blown away by evidence after evidence to the contrary. Solas' reveal about the vallaslin hurt the most because it signifies the whole, painful truth that she would never want to face about her people - it's the utter deconstruction of the ego, her heart is finally sliced open and at that point I think she would accept anything Solas would want from her if only to fill that gaping hole where her bond to her pillars of strength used to be.
I think I'm repeating myself now. Well, it IS a very emotional scene that had a huge impact so...
I am still pretty torn on the drink from the well/let Morri drink and remove the vallaslin/keep it.
It is hard for my mage Levallan to let go all of the ancient knowledge if Morrigan drinks from the well. Still, she is aware of the geas and being bound like that is a major violation of her freedom. She still believes in elven gods whilst Morrigan ridicules the idea and is not afraid of the immediate consequences. It is hard not to meta game this decision. For instance I do think that Mythal's spirit will be preserved, most likely in Morrigan and I want to keep my options open, i.e. not being forced to stand against Solas (I still might do depending on circumstances but would prefer to make the decision myself).
As for the vallaslin, my girl is pretty much on the fence, that's why I struggle so much. There is good argument to do either, and the lines delivered by Lavellan are strong in both situations. Again, I love the removal scene and how vulnerable my girl is without it, with her utmost belief in Solas and what he represents and then him leaving and her staying with nothing. It had such an impact on me seeing her so naked.
It was hard for me to let Morrigan drink. I wanted to drink because I thought, it's Lavellan's heritage, and to be bound to one of her own gods is no price at all, it is an opportunity to rekindle something that was lost. I agree with the sentiment that it was Abelas' and his companions' hoarding actions that basically allowed the cultural entropy to happen that he so scornfully comments on in the first place, and I wonder at what reaction she would have had after the vallaslin reveal following that. However, I was held back by the info that Solas doesn't approve - in hindsight, it would be kind of awkward to be the lover of one god and a slave to another (who's relationship to your lover is ambiguous at best). As a rogue, I could reject it simply for the argument that Morrigan is a mage and can utilize that knowledge better than my Lavellan ever could, this is true. However, her dismissive attitude makes that decision hard just on those grounds alone; as I said above, my Lavellan was trying to desperately cling to the virtues of her people.
Still, it was kind of priceless to see Morrigan discover Mythal's true nature, and in her mother no less. That eased my mind.