Some people want access to your heart without accepting responsibility for what they do inside it.

Some people want the closeness without the accountability that comes with it. They want to be let in, to matter, and to be forgiven, while treating the effects of their behaviour as somehow separate from the relationship. That combination puts all the maintenance on you. It usually persists because it is comfortable for them and gradual enough that you adjust.

Check who takes responsibility for the impact they have rather than only for their intentions. Access should come with some ownership of the consequences. Where it does not, that is worth naming explicitly rather than continuing to absorb.