I'm a total idiot. A few days ago, I admitted to my gf that I was looking at camgirls...

You're obviously not feeling guilty enough if you did it again without a second thought. Her lack of a relationship-ending reaction probably subconsciously spurred you on. Keep it to yourself so you can know what true guilt feels like. It's your sin to bear.

If you don't tell her then you're just lying to her, and no good relationship can be built on lies.

Every relationship is built on lies. Nobody is honest and being themselves on the first few dates, they're trying to be an ideal partner so they don't get rejected. Relationships are about overcoming lies and differences. If your relationship can't do that, it wasn't meant to be.

what's wrong with showing your body on the internet?! and why on hell did she get hurt?! wtf?
basically, what you did wasn't wrong, and you should try and urge her to share the cam with you.
i'll be waiting...

Trying to be a good person is a lie? I would hate to know what you consider telling the truth to mean.

It's not far off from cheating. The only difference is that there's no physical contact. Emotional affairs are still a thing.

You're falsely assuming that lies are always a bad thing, and truth is always a good thing.

When is lying ever justified? And when is truth ever evil?

A person says to their significant other that they're the most beautiful person they've ever seen. This is most likely a lie, but has absolutely no harmful effects to the receiver or the relationship.

On the other end, a significant other asks how good the sex is. The person truthfully responds that it's far worse than their previous two lovers. The relationship falls apart because the significant other begins to doubt themself and believes their relationship may be built on pity.

Maybe those people are really asking a different question. Asking someone if you're beautiful is usually understood as asking whether or not they love you. But there is still the third option of refusing to answer those kinds of questions, so lying is not necessary. Just like when two people share confidential information between themselves (such as a lawyer and client or doctor and patient), it is not lying to keep private information private, because there is trust between the two parties that the information is to be kept private. On the other hand, in situation like the OP describes, when you cheat on a person, you have already violated their trust in your faith, and keeping that information from them would be lying by omission.