I feel like a horrible person

I (17F) am currently in a relationship with my boyfriend (18M). Ive been with him for about a year and a half, for the most part it’s been solid. We’ve never argued, he treats me well, we get along and laugh, etc. I care and love him as much as a teenager can. But, I think I may have feelings for someone else. My best friend (17F), actually. I’ve known my best friend for about 5 years now, I’ve always looked up to her and care about her deeply. Within the past year I really started to notice my feelings for her and how much I think about her. She’s in my dreams confessing to me, telling me to runaway and live my life with her. What makes it even worse is she’s in a relationship as well (16M), and he’s one of my closest friends. I have no clue what to do, if I even remotely say anything, everything will change. Even if she reciprocates how I feel, which I believe is unlikely, nothing would ever be the same. I keep thinking about if I had just waited for her, if I hadn’t of gotten into a relationship maybe things would’ve been different. Maybe there was a chance at some point where I could’ve ended up with her. I feel like Im losing my mind and I feel horrible for feeling this way. I cant stand the thought of losing my friends but I also cant watch her be happy with someone else. Of course I want her to be happy, more than anything, but it just hurts. Everything is good how it is now, I cant risk losing all of it.