I found out my girlfriend cheated on me and I didn't let her know that I knew till I cost her her job.
Why did you decide to keep it from her and mess up her job?
Why should I tell her that I know she cheated on me?
I am not sure why this got so complicated but you should  not do things that cost someone their job. This is a very revengeful way to act.
I didn't think this would be a big deal.
I understand you are really hurt from your actions, but it was still something you should have been up front with. How would you have felt if you just broke the relationship peacefully instead of making things hard? What if she was supporting her family through that job?