"Even with how far I've come, why do I still sometimes feel like I'm failing?"
- Because you're a human being. It's one of the things we do, along with things like eating, worrying and belly laughs.
- Because it hurts less to pre-empt your own failure than to have someone else do it you.
3 quick thoughts to help with this.
A thought about being on the verge of failure doesn't make it real or true, just like a thought about winning the lottery doesn't make it true or real. It's just a thought.
You're better than you often give yourself credit for. Don't diminish your value or capability when a random thought bubbles up.
Don't beat on failure or cast it as the enemy. You need it. Otherwise, chances are you're just playing it safe and doing things you've already done over and over.