I think you have to separate the issues.
Issue one is your marriage, what time limit have you put on it to sort things out? Have you both tried counselling TOGETHER? Because just sorting your own head out really won't help. Best not to mention the crush, I feel it's probably symptomatic of your misery, whenever my marriage goes through a distant patch, my crushes and daydreams become more instense, to deflect the feelings of misery.
Issue two is working with someone you have a major crush on. You say nothing has happened yet, so if you and she were stranded in a hotel, and the opportunity arose would you take it, even though she has a boyfriend and you are married? If so, you have to ask would you accept the consequences? Because if you want your marriage to work, and want to keep your job, that's a hell of risk to take.
A friend of mine recently left her workplace and got a new job because something would have happened eventually with her colleague, she had real feelings for him, and he felt the same, but she was not prepared to have that be the excuse for her marriage breakdown - her marriage strangely got better once she left and wasn't being tortured by what ifs every day. Coincidence maybe?
For you marriage to have a chance at all I think you need to fully commit to your marriage, so perhaps that means physically leaving the place that tortures you, or, more difficulty, emotionally leaving those daydreams.
I wish you luck, it's not a pleasant situation to be in, very confusing, and I think trying to work through both things at once is too much. If your colleague and you are supposed to be together, it will happen even if you're not seeing each other all the time. Likewise, if your marriage is doomed, it will die on it's own even after 100% commitment to trying.
Working through both issues at the same time will inevitably lead to some bad decision making... (even if 5 years down the line it all works out!)