I absolutely understand you. And the things that PPs say about being so close to the edge, not feeling heard, bearing too much...
My GP recently diagnosed me as burnt out. I didn't even realise it was a thing. But I absolutely am. There is no more of me left to give, and yet when I get more And more And more demands, I feel like something snaps. I haven't got the reserves left to be endlessly calm. I, like you, am naturally incredibly calm, but I think people often take advantage of that, because I CAN do lots for people, but it's always "just one more thing" to them, and I find it hard to say no... so I end up here. Nothing left to stretch to.
I want to do EVERYTHING for EVERYBODY to help them out or make things more enjoyable for everyone, but I hadn't realised the huge cost to myself in doing this.
I'm meant to be starting talking to... someone?! Who my GP is referring me to, but I'm imagining a long waiting list till I get seen.
In the meantime, I'm trying to claw back some time for myself. Trying to say no. Trying to rest. Trying to walk away if I feel like I'm about to be overwhelmed... its actually just been helpful to have someone recognise and point out to me that I'm utterly exhausting myself and need to stop that. It's made me calmer just knowing that there's a reason for it, if that makes sense. Like "no, I can control this, I just need to look after myself for a while, then I can deal with this situation better"...
I don't know if any of that helps or resonates, but thought I'd put it there in case.