Be your own worst enemy
Definition 1 of 2
Pro Tip 1/3Self-sabotage
Refers to actions that hinder your progress, often unintentionally and habitually. 
He's always procrastinating, like he's his own worst enemy.
Pro Tip 2/3Irony and regret
Used when someone realizes their actions led to a negative outcome they wanted to avoid. 
She became her own worst enemy by missing deadlines she set herself.
Pro Tip 3/3Avoidable problems
Implies problems could be prevented if one didn't act against their own interests. 
By spreading himself too thin, he was his own worst enemy.