A break up can hurt more than ever. To have intense feelings after a relationship ends is not strange at all, in fact it is completely normal. Everyone has gone through some kind of break up in their lives. What determines your happiness is how you go about getting over a break up and moving on with you life so that you can have other meaningful and possibly much healthier relationship with someone. Maybe you should try to deal with the breakup in a positive way, by acknowledging that the relationship was not going well. And that if you had stayed partners, you would have stopped each other from finding true love and fulfillment with a better suited partner. What you should do is find closure in that the relationship and realize did not work out for a reason, come to grips as to why you feel the way you do. The worst thing you could do is deny your feelings and try to bottle them up inside. Those feelings will come out eventually, but it will take you longer to deal with them later on. Talk to someone about the way you are feeling your counselor, you cannot go a couple of times and think that is going to help you, you may need to have on going counseling to help you work through the feelings you are still having for this woman, maybe talk to a trusted friend and tell them how you are feeling they may be able to give you some advice as to what you can do to make the transition of being single again much easier. Talk to your ex and tell her that you are finding it much harder than you thought to get over her but don't do it at work call her on the phone or write her a letter, you cannot keep these feeling bottled inside of you it makes things worse.
You need to stay hopeful. Just because you are single again, it does not mean you will be lonely for the rest of your life. Stay hopeful in the fact that, that special someone is still out there for you. You just need to be willing to let go of the past and move on to a brighter future. The only thing that obsessing over your ex gets you is you miss out on opportunities with other women and it holds you back from moving on even when your ex may have moved on you are still there pining for her. Instead try to fill your life with things that matter like hanging out with good friends or traveling to different places for a change of scenery or try dating again, a variety of women. Whether they're friends, lovers or someone in between, it's important to have an active life something that will keep your mind off of your ex outside of work. Find hobbies or volunteer just find things that keeps your life busy.
As far as working with her try to avoid contact with her be cordial but out of sight if you run into her speak to her but don't make general conversation especially if you are trying to get over her. Wondering if she has moved on will get you no where but frustrated and obsessive thinking so try to think about other things or put yourself into your work. If possible ask if you can move your desk to be further away, this is the problem when you start relationships with people you work with so try to find someone outside of work from now on to date. If this break up if fresh then you must find closure even if it's meeting the ex for lunch and talking about how you are feeling and how you felt about the breakup and why it happened.
First you need to tell her that it's not going to work and it's best the way things are and also to mean what you say if you do not truly want to break up with her then don't if you are still thinking about her it's for a reason so in order to control how much you think about her you are going to have to figure out what that reason is. Like I said in my answer before first you are going to have to either ask to have your desk moved so that you cannot hear her phone ring or put yourself in your work other than that you are going to think about her especially if you have unresolved feelings for her still. In order to stop thinking about her you need to deal with and then let go of the unresolved feelings especially if you are not planning to stay with her. Only when you are able to find closure and let go of the unresolved feelings will you find it easier to think about her less and obsess about her. Other than leaving your job the tips I gave will help your to work through whatever feelings you are having for her after the break up, break ups are tough on your mind. It's simply impossible to get your ex out of your head. Everything you see and a lot of your surroundings will remind you of her.
Thinking about your ex too much is not healthy, as this will ultimately interfere with other important aspects of your life. You'll need to make yourself have fun if it's a friendly outing, but the main purpose of doing this is to distract your mind from something other than your ex. Don't be shy about meeting new people either, they can really help ease the pain after a break up so you can move on. Until you find the will power to let go of the memory of your ex and get out of the house and back out into the world again you won't get any further to forgetting about your ex and the constant reminders will always be there.