Mother is an ex-Catholic

[0Scarlett_nidiyilii]

I’ve heard many people say that it’s my duty to stop having such a deep bond with my mother because she’s an ex-Catholic.

Wrong.
Shunning isn’t a Catholic practice.

If she were giving you all sorts of crap over converting (not just simply polite disagreement, but harassing behavior, cutting you out of the family dynamic, etc), then you have the option of distancing yourself from the situation.

Because of behavior, not because she’s an ex Catholic.