(one's) own worst enemy
Causing complications to one's life and/or keeping oneself from success; self-destructive or self-sabotaging. Tommy's becoming his own worst enemy with the way he's been skipping school lately. She always sabotages a relationship when it's going really well because she's her own worst enemy.
*one's own worst enemy
Fig. consistently causing oneself to fail; more harmful to oneself than other people are. (*Typically: be ~; become ~.) Ellen: My boss is my enemy. She never says anything good about me. Jane: Ellen, you're your own worst enemy. If you did your job responsibly, your boss would be nicer.