<human>: Can you condense the text into three sentences or less?

I (23f) and a coworker of mine (22m) have been talking and seeing eachother outside of work occasionally for two months. It is mostly an FWB situation, but lately it feels as though he has been developing some feelings. I have as well, and I think it's time to pull back from this situation because of that. He has a long term girlfriend (for shame, I know) that he intends on marrying, and I do not want to be the reason for the dissolution of their relationship or add to any problems they are already having (which they must be, or he wouldn't be messing around with me, right?). I do not have any illusions that he would ever break it off with her to be with me, and I went into this with eyes wide open. We work the same schedule in a small area and have no choice but to continue being around eachother as our shifts are on a rotating basis. Changing shifts is not an option. I don't have a problem with continuing to be around him, but I doubt he will feel the same way. Any time I bring up ending the affair, he gets upset and expresses that he in no way wants to stop what we are doing. I also know that he is firm on the stance of not leaving his girlfriend. Again, I'm okay with this and wish their relationship the best (I don't know her personally), but I feel terrible about this situation now. I know I was in the wrong, and so was he, to do anything like this. Say what you will on that. I'm just trying to figure out how to end this tactfully and with the least amount of awkwardness possible, although I know some amount is unavoidable.
<bot>: I (23f) messed around with coworker (22m) with a gf; feelings present, no intentions of leaving gf; need to know how to end this tactfully. Feels bad, man.