I can talk to my cousins for hours online but I freeze completely in front of them

I need some outside perspective on this because I can't work it out on my own.

I've got family staying this week. Some of them live far away, some are from my home city. It's a full house.

Here's the part that doesn't make sense to me. The cousins I'm closest to are the ones who live far away, and we're in voice chat all the time. Not even gaming, just chatting and chilling for hours. No problem at all. But now they're actually here, in the same room as me, and I go completely blank. I don't know what to say or how to say it. I can't even speak loud enough for the room to hear me. Same people I talk to for hours online.

It's only at family gatherings. Anywhere else, with anyone else, I can talk to people fine.

I think a lot of it is fear of saying the wrong thing. Every sentence gets checked in my head before it comes out and basically nothing makes it through, so I just sit there silent. I'm also exhausted the entire time, even though I'm doing nothing but sitting around, which makes no sense to me either.

There's also one relative who has to be the centre of attention constantly, and a handful of people who talk straight past me when I do say something. I know it's only a few of them but it sticks with me way more than the ones who don't.

I've tried. I've suggested things we could do together, but nobody took me up on it. It's hard to get anything going with a big group. It's got to the point where it feels like nobody wants to spend time with me one on one, though I'm aware I put it out to everyone rather than asking anyone directly.

The thing that frustrates me most is that I've achieved plenty of hard things in my life. I don't understand why I can't get past this one. I don't even know what to say to myself about it.

Has anyone dealt with this? Specifically the thing where you're close to people online but freeze up the second they're in front of you. How did you get anywhere with it?