Having seen my father use references to his exes as a form of emotional abuse toward later partners, I have very mixed feelings about this. If I were spending time with someone who kept centering things with an ex during interactions with me, I'd probably react badly because my experience with it is having seen my father weaponize it.

I think that the LW's situation is different. I'm not sure that the ex is all of the issue leading to wanting to talk about 'I' instead of 'we' because 'we' doesn't have to be just 'my ex and I.' For the childhood stuff, the ex is there, but a 'we' might well include a much larger group that included parents, siblings, members of the congregation or the church youth group. Those things can be talked about with 'we' without most listeners assuming that the comments are connected to the ex personally.

The LW having parted ways with both spouse and the 'conservative Christian culture' in which they grew up implies a break from a much bigger part of their past than the marriage. I wonder if their parents, siblings, other friends, etc. are still available to the LW. Leaving that sort of community can rupture the majority of a person's relationships, so I'm wondering if the ex is all that the LW is trying to verbally separate from.

It could, though, just be the LW not wanting the new partner to think that the LW would prefer to still be with their ex.