I'm seething.
NO is a complete sentence.
You don't have to feel guilty.
You don't have to negotiate.
You don't have to explain.
It doesn't matter if it's family, friend, partner, stranger, anyone. No means no and it doesn't require any explanation and you don't have to feel bad about it.
Say no. Let it go. Your life. Your way. No problem.
There. Unasked for advice.