Given the lack of direct antonyms, you may need to widen your search into the realm of the figurative.
For example, when a person cares about someone, they are often said to be soft on them. Conversely, uncaring, indifferent people are said to be hard. As such, synonyms for harden work as antonyms for care. Here's an example:
- Sorry to firm too much.
Similarly, uncaring people are often characterized as cold. Thus, synonyms for cold or becoming cold can be used as antonyms for care.
If we combine the harden metaphor with the cold metaphor, we might arrive at:
- Sorry to freeze too much.
Lastly, caring people are often characterized as open. As such, antonyms of open might be considered synonyms of uncaring. Here's an example:
- Sorry that I close too soon.
Given that your context is a poem, the open-ended suggestiveness of such metaphors might be desirable. That said, it is hard to hear your desired reading in these metaphorical lines. But maybe a poetic context could push them in that direction.
Or you can recognize that the metrical problem you've found yourself with is more trouble than it's worth. Maybe cut your losses and rephrase the lines.