You're in love. Great. How does he treat people he loves? You have clear evidence of what he says about/ how he treats his girlfriend of five years. Why do you think he'll treat you any better?

She also already knows something is up, and he has no self-control, so good luck keeping this from being a problem in your professional life/ getting around the office.

You feel like a terrible person because you are acting like a terrible person and have done terrible things that are hurting someone else (multiple times). Also how do you know she is "crazy." Literally the only thing you know for sure is that this man has the capacity to lie to people he claims to love. If your only evidence is his word you're a fool. If your life ends up a living hell it isn't because she is crazy, it is because you made irresponsible choices where the most predictable outcome would be your life becoming a living hell.

Stop saying you lack self-control because you don't.

Even if he didn't have a live in partner of five years, someone who tells you they are in love after working with you and a few makeout sessions doesn't actually understand what love is. The fact that you said it back makes your own immaturity that more more obvious. You love a cheater who you haven't even yet built a relationship with and who is putting your professional reputation and emotional wellbeing at risk. Do you see how absurd that sounds? Even without the girlfriend there are red flags all over. Even without the girlfriend I'd be telling you to slow down on dating someone you work with.