Self-sabotage refers to behaviors and patterns that undermine your own goals, relationships, or wellbeing. The defining feature is that the behavior works against what you consciously want, and usually without full conscious awareness.
Self-sabotage is almost always a protection strategy. The behavior protected you from something in a previous context, from rejection, from the weight of success, from intimacy, from exposure. That protection is no longer needed in the current context, but the behavior persists.
Common forms include undermining progress right as success approaches, creating conflict in relationships that are going well, procrastinating on things that matter most, and avoiding opportunities that could lead to growth or visibility.
The behavior often has a justification that feels legitimate: the timing was wrong, the opportunity was not quite right, the relationship had problems. The justifications are real. The underlying function is avoidance of something the self-sabotage protects against.
Changing self-sabotage patterns requires identifying what the behavior is protecting against, which requires honest self-examination of the fears and beliefs that activate it. The behavior changes when the underlying need for protection is addressed.