Problem with a work colleague

Hi all, apologies in advance for the long post, just looking for some help or advice here regarding a problem I have at work. I am in my early 40s, single most of those years, never really in a relationship that lasted more than 6 months, I consider myself a bit of an introvert and prefer my own company most of the time, very shy and maybe suffer a bit from social anxiety.
Anyway about 6 months ago a woman started working in my department and we hit it off straight away, she's eastern European and was pretty new in Ireland at that time, because her English wasn't the best she would ask me many questions about the job and Ireland in general and we ended up being pretty close.
The more we talked the more we realised we had so much in common, sports, jogging, TV shows, reading, hill walking and much more, about a month after meeting she gave me a lovely card and present for my birthday which I thought was strange but assumed was their custom.
She is married but divulged that she is very unhappy in her marriage and only really staying with him because of their kids, aged 19 and 15, there is no love there at all according to her.
On hearing this I backed off a bit, didn't really want to be getting involved in a situation like that even though there seemed to be an obvious mutual attraction. We do shift work so could go 4-5 days sometimes without working together or seeing each other, usually at this time she would text me and I would text back, this could go on for hours from gossip to friendly banter to sexting, now I am guilty in my involvement but it was always her who made the first move.
Sometimes she would suggest meeting up then in the next sentence say she's confused and afraid, this would only happen by text, when we would meet face to face at work it would only go as far as friendly banter. We never had any physical contact, no kissing, sex or anything like that.
This went on for a few months, I wouldn't have a huge amount of friends and she was my closest friend during that time, we would talk to each other about everything and anything.
Then the ghosting started, subtle at first, I thought she was maybe finding her feet at bit at work, getting more independent and not needing my help anymore, still got texts but not as much, slowly but surely I noticed she was starting to avoid me, I asked her about this and the reply was, so busy, so tired, no time to talk etc.
So for a few weeks I tried to avoid her, forget about her but to no avail, it's just killing me, everytime I see her she's talking to someone, smiling, joking etc. but once I approach her the shutters come down, she did on one occasion say to me that there could be no future for us and that she won't be in Ireland long term, also that I seem to be obsessed with her, maybe this is true but she took the lead in this area, I only played along with the sexting etc. after she started it.
While I know there would never be a relationship between us I just hugely miss her as a friend, she seems quite good at the mind games and is wrecking my head, she was meant to go home to her country for 3 weeks recently but flight was cancelled, I was looking forward to my head getting some rest but that won't be happening now.
I know for most guys it would be easy to say "f*** her, move on and forget about her" but I don't think I'm wired that way, when I see her it's like a dagger in my heart, I just want to go back to when we used have to lovely chats and the banter, it has gotten to the stage where I now hate my job, a job I used to like but now I look for excuses to get out of work.
I know I sound like a bit of a fool here, many I am too gullible, maybe it's not my head doing the thinking, any advice would be greatly appreciated