It's both — it's a feedback loop. Here's a typical couple of days when you're dealing with uncontrolled anxiety:
You feel like shit, that makes you unproductive and slow and dull. End of the day, you didn't get anything done because you stare at your text editor and your brain just refuses to engage in anything but panicking over... something. You get home, Now you still fill like shit the same as before, but also feel like shit over the fact that you didn't get anything done today and and you need to make up for it tomorrow. But then you don't get any proper sleep and you still have to catch and on lost time and you're in for a pretty bad day, but you shouldn't think like that, why are you wallowing in self pity, just get over yourself and get stuff done. But that doesn't really work, oh shit oh shit oh shit thingsarespiralingoutofcontrolwhatdoIdo?
You fall into that trap and now you're proper screwed. Anxiety is causing problems, problems are causing anxiety. The alternative is to learn to cut the second order effect short, which stops the loop. Yes, you had a bad day, and yes, your project is delayed because of it. What's done is done, and you can't fix it, so just accept it. Let people know you're behind schedule, replan if necessary, and move on.
In many ways, breaking that cycle is an exercise in allowing yourself self-compassion without indulging in self-pity.