I fell in love for a friend

Hi everyone,
I know it might sound retard and "overasked" in some ways, but I actually fell in love with a friend of mine. I met her 4 years ago because she was the girlfriend of a friend, and they broke up one year ago. I used to hang out with this guy a lot because we lived in the same town, but then 1 and a half years ago he went back to his hometown, so I really didn't get to know her better and have meaningful conversations until they broke up last year september. We have been going out weekly (as we have the same circle of people) for the last year, and when we met we would always talk freely and had fun together. I always had the feeling that she liked being around me but I didn't really went too far with advances as I would feel guilty as she still is my friend's ex. Then six months ago she moved to another town, and after a while we didn;t see each other, she came to visit me in my home town this summer, initially for three days, but she stayed almost three weeks, and we had an amazing time, but I didn't know what to do, I was too afraid of being inappropriate since she was also my guest, but we really got closer as we also had been talking about really deep things regarding our lives, and since then we have been texting and calling each other daily, sometimes i really would like to text her or call her for no reason, but at the same time i tell my self that she is not my girlfriend. The thing is that she came last week in the city where I live for 1 month and a half, after that she will move to another country, in another continent, maybe for a while, maybe forever, and I don't really know if I am going to see her again. The thing is that we have also built a lot of confidence with each other, I know she trusts me as she always says like "I feel safe around you" and makes compliments about me, the only thing that is holding me in kissing her is that I am afraid I will let her down because she would think that I acted this friendly just because I wanted her, but the truth is that I genuinely never thought that I'd be ending up liking her this much, I had actually tried to be as rational as possible but during those days together something happened, i don't know what, and I realised it the moment I left her at the airiport, the greatest sense of emptiness I had ever tried, I couldn't look at her eyes when she was leaving cause I would had just exploded in tears. Now we have seen each other daily but just for a coffe,or went to the bar friday, but I don't understand where is this going. Do you think i should just tell her what i really feel? Or forget it and act like evruthing is normal until she lives ? But what if the remorse of not having done it will kill me for life? i am really confused guys thanks for any advice you can also ask me more details so you'd get a better picture . I always think about her it's crazy.