To regress; to slip backwards or revert to a previous, worse state.

“He felt better for a little while, before his condition started to backslide.”

To shirk responsibility; to renege on one's obligations or commitments.

“Rich countries are backsliding on their commitment to agree to new WTO measures to help people in poor countries gain access to affordable medicines. — Oxfam press release, 24 June 2002”

An occasion on which one backslides, especially in a moral sense

Sliding back