You're confusing what they did not deserve, it's not the truth that they don't deserve, it's the cheating in the first place that they didn't deserve.

Not confusing anything, it's the pain you will cause by being selfish twice, once when you cheated (this pain you can keep to yourself), and again selfishly telling your partner you cheated so you can feel better (a pain your giving to someone else)

So, pretty much everything is selfish then.

It's selfish that you cheated, and selfish that you passed on your guilt 

What is self sacrificing would be to commit to never doing it again, and living with your own guilt as it's your own responsibility 

there was a time when i was so addicted to this mmo that my relationship suffered because of it. I didn't give her the attention that she deserved, and looking back on it I don't think I would have blamed her if she had cheated. But at that point it would have made more sense just to end the relationship. But of course that wasn't the only issue in our relationship, and we broke up anyway.

Telling them could destroy your relationship with them though and make them hate you, so it's not like telling them the truth is just full of advantages for yourself.

No it's not, but in some odd way people don't want the responsibility of holding that guilt and feel better when they tell someone, which in this case can hurt an innocent someone very badly  

You already broke the commitment though, either face up to the punishment or try to work things out.

We as humans break things all the time often by mistake, life is messy, it's your choice of cause to abandon the relationship, but if you just made a mistake once and really love your partner you would be a fool to really destroy your and your partners chance for happiness, but if your a serial cheater then to hell with you  

If your partner cheated on you, wouldn't you want to know?