should I tell her I like her?

I have a huge problem I need help with. I'll start at the very beginning of the very long story. almost 3 years ago, I met a friend who we will call L. L and I were good friends for about 2 months when we met a girl will we call K. L immediately started flirting with K, because he obviously liked her. I liked her too, but I am the shy, and quiet around new people type. So i just let him flirt and kinda stood there awkwardly. I never told L that I also liked K. They started dating shortly after that day, and the 3 of us would hang out every day for about 2 years. Almost right at the beginning of their relationship I resented L, not only because of K, but for many other reasons as well. I stayed his friend only because he was K's boyfriend and if i wasn't friends with him, I couldn't be friends with her. I had dated one girl, J, during that 2 years. She is K's best friend. She tore my heart to shreds and now, after about 2 and a half years after we broke up, I am basically over her. After the most recent valentines day, due to severe depression and physical pain I began using heroin on a daily basis. I had also done crack a few times. K voiced her concern for me and told me I had 2 weeks to quit or I would lose her as a friend. During that 2 weeks, K and L broke up. That same day I told L that we were no longer friends, because he was a very bad friend, and the only reason I stayed his friend was so that I could be friends with K. Because of the reasons mentioned before, i continued to use drugs. K then told me that she could not watch me hurt myself, so she stopped talking to me. I was still friends with J, even after what she put me through. After about 2 weeks after K stopped talking to me, I forced myself to get clean, no rehab was necessary, just the thought of having K as a friend again was enough to get me clean. (That shows how much I care about her.) So I told J that I was clean and asked her to tell K. She did, and K and I are now friends again and have been for about 3 weeks. There were only about 2 and a half weeks that we did not talk, so our friendship picked up where it left off. We hang out almost everyday, and laugh and have a lot of fun. We have been such good friends for almost 3 years, and I have always liked K. But ever since we became friends again my love for her is growing by the minute. Every time I see her I feel, happy. And i'm always depressed. Her being around even diminishes my physical pain often times. I think i'm in love with her. But she is my ex friend's ex girlfriend, and my ex girlfriends best friend. K often makes it a point to say to me that I am her best friend ever, and that she will always be here for me. She is my best (and almost only) friend. She is the one I turn to when i'm upset and vice versa. I don't want to tell her I like her because I don't want to ruin our friendship. But what if she likes me back? What if for the past 3 years she has felt the same way? I need advice, I don't know what to do or how to do it. Please help