Fell in love with a co worker who is in a relationship

Hi guys I really need some advise on what to do or say also from women who will probably know what the girl I'm in love with is thinking.
So for 3 years I have been working with this girl 40 hours a week 8 hours a day we are together we only work together and most of the time are both off at the same time as we can't work when one of us is not there.
When I first started working there I was in a relationship but knew this girl liked me we had a few intimate moments nothing to serious. She then got into a relationship and as time went on we had so much fun times and moments I fell in love with this girl and left my girlfriend.
The girl I work with has been in a relationship for almost 2 years but in the last 8 months things have got crazy between us. It all started when we met up alone one night and got drunk I told her how much i love her and that i want to be with her she didn't believe me at first but now she does. That night we ended up in a hotel both carried on drinking. Then we kissed and done a few more things but she said the guilt is too much she can't carry on so we both went to sleep like a couple that have been together for years. After that she told me she's in a relationship and that it can't happen again because the guilt will ruin her and make her ill as she has suffered from depression before. But it did happen again several times every time out of work when we are alone it just happens we kiss and touch each other in all sorts of ways. Sometimes we be in each other's company for hours and hours after work even though we spend all day at work together.
This keeps on happening and I said it's not fair on me you need to make a decision either tell me to go away or be with me. She told she's in a relationship but can't bear for me to go away and she cried telling me not to leave her.
One night she was drunk with her friends and called me because she couldn't get a cab so I went with a friend who drove as i was also drunk to pick them up again same thing she told me I have messed her head up and that she loves her bf but then we started kissing holding hands etc. Everytime we get a chance we are holding hands like a couple and touching each other in intimate ways. I tried dating but I can't I love her. When i told her about my date she just says you have a girlfriend now good for you and pretends like she's happy. Her bf lives 40 miles away and is a waste of space he still lives with his parents. Every times she's not there with him she's txting me all day till the early hours and us calling each other.
She says we can't be together after work because of what happens yet we always do we can't help it. She says the guilt of cheating is too much and that she does love her boyfriend.
BUT I ask why do all this with me? Why ask me to stay and not leave or stop talking to you when u love him. Her answer is "i dont know" and that she doesn't have an answer. She asked me that question and I told her I love her I knw my reason.
Yesterday when I argued with her she asked me why i always do this just before she goes to see her bf I told her it helps me get through the days we don't talk and that i miss her so much she also said she misses me and knows how much i care about her but thinks my love for her is exaggerated I told her she has no idea how much i love her.

I just don't knw what to do. All I do is think about her all the time she's on my mind constantly and I have shutdown so many girls who want to be with me because of her. I can't be with anyone else and seeing her with someone else is killing me inside.

All she says to me is you get so much attention all these girls want you at work and out of work. But I told her I want none of them. Only her.

I really don't knw what to do.