Caught Between My Job and My Man

So this has been eating away at me for a while now. I've got a great job as a barista at a cozy little coffee shop downtown, where everyone knows each other's names and you get to chat with regulars about their day. And let’s not forget my boyfriend of three years, Mark, we’ve had some rough patches but overall it’s been solid, with good communication and all that jazz. The problem started when I began noticing one of the regular customers, Alex. He comes in every morning around 7am for his triple espresso and chats me up like he owns the place. There's just something about him, maybe it’s how confident he is, or perhaps his casual charm as we exchange pleasantries daily, but it made me feel things I shouldn’t. A few weeks ago, Alex started leaving tips that felt more like gifts: little trinkets from a nearby store, hand-written notes on napkins about his day. It escalated to the point where he began staying after hours for 'late-night coffees' when my shift ended, and once I even found myself walking him home in the dark. It didn’t stay platonic though; one night, we kind of crossed a line that I’m not sure is coming back from, it felt so wrong yet exciting at the same time. Not going into graphic detail, but let’s just say there was no mistaking what happened. And now, here's where my head’s been spinning: I still have my job, and Mark and I are doing fine on the surface, but this... thing with Alex has me questioning everything. How am I supposed to go back to work after that? And if Mark finds out, would he understand or see me differently? I don’t know what I want anymore; part of me wants to throw everything away for whatever could be between Alex and me, while the other side wants to cling onto stability with Mark. So here’s my question: Am I being unreasonable by thinking about this at all? What should I do moving forward? Your thoughts would mean a lot.