Let's look at these thoughts...

They ignored me! I thought they were my friends!		So, you are saying that since these people ignored you, they must not be your friends - right? If this is true, why is this a problem?
I guess I am not cool enough to be their friends.		So you are saying that they are not your friends because you are not 'cool' enough? If this is true, why is this a problem?
Because then nobody will want to be my friend and no one will like me.		So, you are saying that other people will see that you are not cool and they won't want to be your freinds, either - right? If this is true, why is this a problem?
Because if nobody likes me, then I am a loser.		So, if nobody likes you, then that's what a loser is? And nobody wants to be a loser, right?
Right