Most guys try to hide performance anxiety and hope it doesn't show up. Silence feels safer. In practice, it's the thing that turns a small moment into a bad night.
What she assumes when you say nothing
If you lose your erection and don't say anything, your partner has to guess what happened. She almost never guesses "he's nervous." She guesses that you're not attracted to her, that she's not enough, or that you're bored. Once she feels rejected, the room gets heavy fast, and sometimes she gets upset or cries. Now you're both spiraling.
All of that can be headed off with one honest sentence.
Keep it short and light
This isn't a confession or a therapy session. Before anything sexual gets going, say something simple and casual, along the lines of:
"Hey, sometimes it takes me a little longer to get going with someone new."
Or: "I get in my head sometimes. I'm really into this, so if it takes a minute, just ignore it."
That's it. The book's rule is to keep it honest, simple and light.
Why it works so well
When she knows what might happen, she stops watching you for clues. She stops taking it personally. She relaxes, and when she relaxes, you relax. You also remove the "first time pressure," the feeling that you only get one shot with this person and if it fails you're finished. You get room to breathe and room to try again.
Saying the thing you're most afraid of out loud also shrinks it. It stops being a monster in your head and becomes an ordinary thing two adults talked about. Once something is ordinary, it has much less control over you.
If it happens anyway
Don't go quiet and don't disappear. Stay close and calm. Tell her she's attractive, that this is something your mind does sometimes, and that you want to keep going. Then ask if she wants to keep fooling around. The book points out that the second attempt after an honest conversation very often goes well, because the worst thing already happened and she didn't leave. She's on your team now.
Being upfront doesn't make you look inexperienced. It makes you look emotionally intelligent. Women tend to judge the man who goes silent and weird far more than the man who's honest.