It seems like you have a pattern of not being able to walk away from relationships when they are very clearly not working and badly damaged. You cling, even after they are essentially over in all but name.
And from a larger perspective, it sounds like you like the idea of having a girlfriend/partner, and you like the idea of who you make these specific women out to be, but you see that far more than you see the reality of them. You love the idea of who you think they are, not who they actually are. You love the idea of a relationship and try to shoehorn the relationship you are in, and the woman you are in it with, into that vision, rather than going out and actually finding something close to the vision. "Insert nice woman into girlfriend role", rather than developing a healthy, stable, relatively drama-free relationship organically. I think maybe you want it so badly that you are willing to tell yourself you have it, repeatedly, when you clearly don't.
You say she "led him on" to keep the job. If a woman has to let her boss think he might get in her pants again in order to keep her job, that's abusive, disgusting, and illegal on his part. A woman doing what she has to in order to not be fired for not sleeping her boss is in a shit situation, doing the best she can. She is not "leading anyone one". She is dealing with intense sexual harassment and is trying not to lose her livelihood. And the fact that you don't acknowledge that are are upset about it doesn't say great things about you, and is something you need to explore. On your own. ruly is.
Meanwhile you were "talking to other girls" for a year. That can mean a lot of things but it sounds like at best it was poor judgement and disrespectful to your partner, and at worst it was an emotional affair.
Walk away. Start therapy ASAP, and maybe make a rule for yourself that you absolutely will not date anyone for at least 6 months (and that you need to be doing counseling that entire time). You've got some stuff to work through and until you do, it is very unlikely you will ever be able to find a healthy relationship or be the partner that someone healthy and stable needs you to be.