I think the focus of your issues should be your current relationship, not what is going on with the coworker you are attracted to.
The problem with dragging it on for so long until you completely lose feelings is that it's going to hurt worse in the end. More for her. Being stuck in something you don't want to be in is basically a form of passive aggression, because you know that you aren't giving that person one hundred percent. It's wrong.
I'm not saying that at the first whiff of something you need to "have the talk", but if it drags on for weeks and months of the same arguments with no solutions, where is it really going? Down. And the longer it goes down, the uglier it gets. If she doesn't want to work towards a solution or you don't want to, it's just going to go down. Sweeping it under the rug is the immature way to handle it but a popular way because most people "just don't want to fight anymore."
You have been emotionally cheating on your girlfriend, and you know this. It makes you feel shitty too doesn't it? That's why you have to have enough security in yourself to end it and be on your own before anything like this happens. You are keeping your girlfriend around as a cushion and it's not right.
You and your girlfriend have been dating long enough and her insecurities are pouring through the floodgates. I think you know that she needs to work on herself and her immaturities and inability to deal with stress and being apart might be the best thing to happen for her. I know you feel like as she has said "die without you", but she really needs to suck it up. She needs to understand that she doesn't need anybody in her life to get by and that you are more than just a crutch for her during tough times. We all go through difficult points in our lives but we want to come out stronger on the other end. That's when you know both are at the same level of commitment. And you guys clearly aren't.
You need to focus on your crumbling relationship first before you are completely mentally checked out. That is the mature adult thing to do. Not chase after another girl and use your current girl as a stepping stone because if you do that, you will not learn a damn thing from your mistakes that you made in this relationship and it will haunt you with the new girl. That is a promise.
Waking up next to a beautiful girl,
Step outside and say hello to my beautiful world.