1) Terry starts out the gate with "you didn't treat me like I'm someone you love. You didn't even treat me like I was a person" (which was Viren's damning sin with Lissa). The fact that this threatens Terry's ability to be himself — the thing he's put above everything else since childhood ("but I knew, I always knew, that I was a buck", and held steadfastly to ("I'm going to be strong enough to do whatever I need to do and have feelings"), all of which are rooted in an intrinsic emphasis on agency ("I chose the name Terrestrius") is like... yeah, his relationship with Claudia as a counter balance never stood a single fucking chance.
2) Terry leaves to escape being further changed, but part of the loss of his true heart (which does canonically happen) in addition to realizing what he's been complicit in means that he knows he's already been fundamentally altered. Is he still the person Claudia loves? On a certain level yes, but on another level, it's already a no, and only continues in that vein (given 7x06).
3) The fact that Terry is saying "If I stay with you, I won't like myself, and I won't be the person you love anymore" is just another devastating cherry on top, especially in contrast to Callum accepting that Rayla needs to leave in 4x09: if you only love your partner when you've demanded or enabled that they change beyond recognition, do you really love them at all?
4) There are two interesting ways we can, therefore, read Terry's final line. We can read it as a continuous sentiment (which I think is the intended takeaway) of "If I let myself change to someone I don't like, I'm not really the person you fell in love with anymore" as something that has not yet wholly happened, and contingent on the if. We can also, thanks to the trail off, read it as him grappling with "I'm not really the person you fell in love with anymore, am I?" as a change that is happening in the scene itself, because he's changed (and so has, of course, his perception of Claudia and his willingness to be with her). and I think that's neat