I fight the urge to cheat everyday!
What has been the reason that you have not done it.
I do not want to get caught and have my partner leave me.
I think that's a mature attitude.  You're right that it could cause major relationship issues and you should continue to resist the urge to cheat.
I'm craving sex so much. I want to fuck someone.
It is good that you do not want to cheat for the sake of your partner. If you feel you are craving sex too much than you should talk to your partner about it. Hopefully your partner can start to have sex with you more often.