Struggling with Last-Minute Changes Due to Autism and Trauma

I feel like the inability to cope with last-minute changes caused by my autism and trauma is making my best friend/flatmate grow tired of me. He asked if, when his partner comes over next, they get too tired to go home, can they stay over at the last minute? And like, I want to say yes. I like his partner, but it's not because its them, it's a general thing: any last-minute changes, and I have a complete breakdown. I'm 24 now, and I feel like I can't stay like this, I need to get to be okay with these sorts of things, these things aren't a danger. But apart from it being an autism thing, I have absolutely no idea why it affects me so much.