Yo, don't worry about. It happens. Sometimes I'm stressed, tired, whatever. And condoms don't help. Plus I'm getting old ;)
The #1 thing is to not care. Sure, you want to fuck her, but if you start worrying that you're not going to get hard, then guess what: You aren't going to get hard. This is probably why the 2nd case happened.
I found what finesse said really funny, because girls get really self-conscious about them not turning you on enough.
