Kinda slept with my best friend (need advice asap pls!)

I’ve been best friends with this boy since we were 17, now 20 and 21 (let’s call him J). We instantly hit it off, had everything in common, always felt safe and comfortable around each other, etc. We agreed from day one that we’d only ever be STRICTLY platonic. We were true to that, we never got sexual, never disclosed our relationships with others, never once talked about/ felt being more until two days ago. He’s been single for about two years now and I’d found out my boyfriend of two years was cheating one me. I went to J’s house to cry, vent, sleep, get advice, etc. We got a bit crazy on a emotion and sleep deprivation when he told me he’s always loved me and wanted to be with me and he didn’t want to settle for someone else.

I was, and still am a bit shocked by this. I NEVER got the impression he wanted to be more than friends. But okay when he told me he started to lean in to kiss me. I didn’t really know what to do so I just let him and even kinda went in too. I was honestly so emotional, heartbroken, felt insecure, etc. I needed that kind of affection from someone who truly loved me. We ended up having sex, but it was different. Like you know when you have really slow passionate sex with someone who you truly trust to take care of you? Like it’s actual love not just sex. It was like that. I don’t regret it, but now there’s a bit of drama.

He thinks we’re together now. Like we woke up the next morning and he just wants to kiss me, make all these date plans, say I’m his girlfriend, claim we should move in together, etc. I love him, but I’m still heartbroken about my ex and in no position to be in a fair relationship! How do I tell his this without hurting his feelings or ruining our friendship?