ummm... are you talking about wanting to cheat on your girlfriend? just clarify for me please.
and yes the anxiety you experience is normal. well whenever you tried with this girl when you and your girlfriend were on a break, i'm sure performance anxiety was affecting you. Also perhaps you deep down felt a bit bad for being with someone different, even if it wasnt cheating. maybe you still had feelings for the girl you are with now.
there's nothing wrong with you, if that's what you're asking. you're just comfortable and at ease with your girlfriend. that's how it is when you are with someone for a while. being with anyone new is always strange and daunting at first.