You're worrying because a) you're human, b) charge is scary, and c) you're human.
Here are some facts for you:
No one walks in the door knowing a code base.
No one is proficient at a code base after one month.
Imposter Syndrome is real, and lots of people have it.
Here is some emotion for you:
CONGRATULATIONS! You've made it this far, that's really awesome! The job search is so difficult, especially for those of us with depression and anxiety. Are you proud of yourself? I am.
It's okay to be worried, but don't be down on yourself. Let's imagine that you get in the door and you can't hack it - worst case. (But not the most likely case.) Then what? You'll struggle for a few months. You'll probably say really awful things about yourself. Then you'll either a) get it or b) not get it. Let's go with b, again, worst case. What is the worst thing that could happen? They could fire you? That's not really likely. If they've invested in hiring you, they're usually going to put a little effort into keeping you around and finding a way to get you up to speed (unless it's a poor startup). They could call you out, publicly, and shame you? Also not likely. You could decide to leave and find a job where you don't feel awful at the end of the day. That's not so bad, so it didn't work out, that's sad but not the end of the world.
But really, what's the most likely situation? You get in the door, you're list for a few weeks, but you gradually pick it up. In a few months, you're starting to feel confident and know your limits. In a year, it's old hat.
Have you ever "sucked" before? Have you ever not been able to figure it out?
And really, if you can't figure out the code, that's usually because it's written in an unclear way, which is a reflection on the writer, not on you.