So lately, a coworker and I have been getting closer emotionally and physically and she still has yet to break up with her boyfriend. She tells me she is afraid to and doesn't want to do it before Christmas, but would soon after and we could date. We've made out multiple times, and then we got high and made out then I fingered her and she gave me a handjob. She then told me how amazing I was, and then says, "Now tell me: how badly have you wanted to fuck me?" but we didn't have sex because it was 3 in the morning and she had work at 11. She then said we would "pick up where we left off" on Monday."
And I just got high as fuck around an hour ago and I thought about and missed her. I realized I'm really falling for her and then it hit me: she's a fucking cheater. I didn't really even take that into account. I know I would feel terrible if she did that to me. I even told her that if we end up dating, and she lost feelings for me, to tell me right away because cheating would break my heart even more.
There's a lot to factor in, but I wanted to know what you all felt about it because it's a crazy love triangle. With a coworker who's cheating for you. I personally am going to keep going after her. Survival of the fittest and hope she remains honest. But if she doesn't it was my mistake. So let me know and stay high. 
I'm saying this from personal experience,
If she does it once than she'll do it again.
I know that a lot of people want to believe that there is good in everyone, but some people don't want good things for themselves. They live a life of self sabotage, and if your around them, it will begin to affect you as well. When they want to stop this behavior than they will take the steps
THEMSELVES.
I know the idea of helping these emotionally and mentally unstable people can be intriguing, as I've done it too many times in the past, but their negativity and instability will begin to affect the positive things in your life.
If you have to tell her anything, tell her that your family is having a "very personal issue" that you don't want to talk about, and that you are going to be focusing more on them.
Remember, people like her are very emotionally and mentally unstable, and you have to tread on eggshells until your at an arms length from the situation.