Having actually been in the position of Girl #1 before (I had a massive thing for my colleague, who had no interest in me, but still I longed), the best thing that happened to me was being told directly that he wasn't interested. For months, I had colleagues and friends alike telling me he wasn't interested, but I still held a candle of hope 'cause he never directly said anything about the issue to me. It all kind of exploded when he started dating his best friend's ex (his best friend was a fellow colleague, so work got kind of murderous, haha), but the thing that really woke me up and finally convinced me that he didn't feel the same was when he took me to the side early one morning at work, told me he'd started dating this girl, and that he didn't want me to remain hopeful that something could happen.
I was devastated, naturally, and so your Girl #1 will probably be, but it was a real wake up call, hearing it directly from the horse's mouth. Two years later, I'm happily in a relationship, and am actually grateful that things happened as they did. She'll get over it; despite her behaviour, she probably isn't psychopathic and probably won't kill you upon hearing you're interested in someone else, but if you leave her to fester, she'll keep believing something can happen. Talk to her and let her know you're not interested, and that you're going to start seeing someone else, and give it a small amount of time, and things should work out fine.
I was devastated, naturally, and so your Girl #1 will probably be, but it was a real wake up call, hearing it directly from the horse's mouth. Two years later, I'm happily in a relationship, and am actually grateful that things happened as they did. She'll get over it; despite her behaviour, she probably isn't psychopathic and probably won't kill you upon hearing you're interested in someone else, but if you leave her to fester, she'll keep believing something can happen. Talk to her and let her know you're not interested, and that you're going to start seeing someone else, and give it a small amount of time, and things should work out fine.