We all have days where we stumble over copy, and can't seem to find our groove when executing a commercial or narration script. It's easy to get frustrated and beat yourself up. The best thing to do is walk away from the project and come back to it later. Too often, talents seem to force a performance when they're having a bad day. I've been guilty of this myself, and it's a waste of time and energy.
The problem is that you can hear the irritation in your voice. You're trying to record a friendly and relatively upbeat commercial, but it's slowly starting to sound like you're pissed off. At this point, you're beginning to waste your client's time, and you're likely going to have to re-record it anyway. This isn't the best method for keeping a client for life.