How do I stop my ex from texting me at night?

So my ex and I broke up like four months ago, and it was kind of messy — we’d been together for three years, living together, the whole thing. We agreed to stay friends, which maybe was a dumb idea, but we share a lot of queer spaces in our community and I didn’t want it to be weird. At first it was cool — occasional check-ins, cute memes, nothing heavy. But now they’ve started texting me every night around midnight, always like ‘hey u up?’ and it turns into these long emotional convos about how they miss me or they’re lonely or whatever. Like I get it, I miss parts of them too, but we’re not getting back together — I’m actually starting to see someone, nothing serious yet but it feels real? And every time my phone buzzes with their name, my stomach drops. I’ve tried not responding, but then they’ll show up at the same brunch spot I go to like a few days later and act like nothing’s wrong. I don’t want to be cruel, but I need them to just… stop. How do I actually say no when they act like we’re still close? What if they say I’m being cold? Ugh. I just want my life back.