all up in my feels, pls help

I started a job recently where i've been working closely with this guy, and unfortunately I've caught feels biiiig time. His laugh, his smile, and how when he smiles big enough, he gets these crinkles by his eyes. Ive been thinking our interactions were innocent enough, until yesterday's shift, where he was more flirty. part of me enjoyed it, but every time I felt those butterflies, I had to keep shoving those feelings down. At one point, he may have lowkey asked me out, not as like a date but maybe not *not* as a date? It would have been for today. And I feel so guilty because part of me really wanted to say yes, but I told him I already had plans... My plans were with my boyfriend for our 3rd anniversary. I know, I'm horrible. I have the weirdest feelings about my boyfriend right now, though, and I've been half thinking about ending it for about four or five months now, but I never had the courage to do it because he's the best friend i've ever had and i would hate to lose that. I'm just not attracted to him anymore and I don't feel the same love I used to. He's great, he's caring and sweet, but he loves me more than I love him anymore and it's eating away at me. He talks about our future all the time like we always have, and it's a safe future, but I just don't think I can see it with him anymore. We almost broke up about three months ago. It was an awful day and my depression was in full swing. I told him I wasn't feeling satisfied with our relationship, we were both in tears, but in the end I told him I couldn't leave him if it meant losing him all together, that he's my best friend and that meant too much to me to lose. So we stayed together, and it hasn't been mentioned since. Part of me still feels that way and wants to stick it out to see if it gets better, and that's what i've been doing for some time now, but I feel like I deserve a life i'm excited for, not just dealing with, and he deserves someone who loves him as much as he loves them. It's been a while since I've had butterflies with him. I don't get excited to see him anymore. I don't miss him as much when we're not together. I forgot that that's important until yesterday, feeling all of that with my coworker. I feel like such a garbage human for feeling this way. It's our anniversary, and I haven't been able to stop thinking about my coworker all day; his voice, his smell, his gorgeous light blue eyes, those creases by his eyes that appear when he smiles big after something I said. I know I'm not being fair to either of them and i'm not being fair to myself. I don't know what to do. I know now that I should end it with my boyfriend, but I don't know how to end the relationship but try to keep the friendship, and I certainly couldn't have done it today on our anniversary. But breaking up with him for potential future happiness could mean losing my favorite person, which I'm not sure I could handle. I need advice, please. What would any of you do in my situation? What should I do? Any sort of guidance is helpful. Sorry this is so crazy long.