I went to post this in relationships but have read enough about that topic to have been apprehensive. Was hoping I could get a more rounded opinion here.
I have known my friend for 4 years and we met while studying together. We had our ups and downs friendship wise but I think that was just us maturing as we are in our twenties now. When we first met he had just come out of a 3 year relationship with a girl he had known since being a teenager and was at a place where he just wanted to party and have fun. I was in an up and down relationship which I left after knowing my friend for one year (not cos of him, just giving time frame). I knew I was attracted to him from the start but it wasn't like a loving or caring feeling beyond friendship, though we were both young and flirtatious, there was always a lot of tension between us and people in our social group used to talk about it alot.
We both started at the same university together and it was fun partying in the first year for both of us as I was newly single and going a bit wild too. We used to meet for lunch 2/ 3 times a week and talk about uni and other stuff. He was always a little funny with me, like he always had his guard up and I used to interpret it as him treating me unkindly from time to time, but we stayed friends and I soon learned it was his way of being, so to speak.
Over the past few years we have essentially grown up into adults together. I told him a few years ago when I started to have feelings of really caring about him that I liked him and he told me he wasn't interested. He said I was a beautiful girl and he respected me and our friendship so didn't want to do anything. I accepted that, but it did cause some tension in our friendship for a few months (we stopped meeting for lunch and talking like before, his behaviour became even more guarded around me).
After a summer break we were back to ourselves and I have always cared for him, a feeling that has gotten deeper and more meaningful as the years have gone on. I know this has been a two way friendship and he too cares about me because (though he is a bloke-ish type and doesn't talk openly emotionally) we have had conversations where it has been mentioned and I am his oldest and closest female friend, and someone he talks to about things he doesn't talk to his other mates about. There have been moments where his feelings for me have been on show, but I don't want to go into too much detail as its somewhat embarrassing 
I met my DP two years ago and we have been together since though have had a couple of breaks lasting two to three months (mainly as I was aprehensive about having another long term relationship). But I love, respect and want to be with my DP and we have maintained a good relationship overall and even a friendship when we weren't together.
My DP has mentioned in the past a few times that he felt my friendship with the other guy is slightly odd, sort of tense and seeming to have more to it from my friends behaviour towards me from a guys perspective. I acknowledged this and told him of our history (have always been an honest and open type) but he knows I would always be honest with him if there was something more to tell, which there isnt really.
They have not really spent much time together though as I am now pregnant and comitting to my DP, my friend seems to be wanting to make more of an effort to get to know my DP which is great for me as they are both important to me and I would like them to be friends.
But there have been funny moments, tension and my feelings for my friend seem to not be going away. He has been around in my life through big changes and we have known the best and worst sides of each other. He is going to be away for a substantial period of time from this summer ending and my feelings of sadness are very great. I have other close friends of course, but there is something different about our friendship. When he comes back I will be settled down and we won't see each other as much and it is making me very sad. Not a day goes by that I don't think about him and it scares me to think that I am actually in love with him. I know he is going to miss me too because we have talked about it, he doesn't want us to loose touch.
I have had him in the friend zone for so long that acknowledging it seems too far fetched but I have to get this off my chest.
I love my DP but feel so guilty and sad all at the same time.
I have so many mixed emotions just writing this, I have a partner and family to think of now and I know the implications of that.
Sorry this is really long, just needed to let this out. I guess I need a reality check and someone to smack some sense into me. I guess I am just wondering if I really am a horrid person, because I really feel like one.