Would you please correct my sentence of English writing below?

(It has been raining all day
so i am about to get depressed)
i am gonna make this sentence as blew one. which one is right?
->[i am about to get depressed ( to rain / raining) all day ]

I've known "To + verb" means not to be acted /
"Verb+~ing" means in progress from past certain time
so, blew sentence is understandable by the theory of "To+Verb" and "Verb+~ing"
[It is impossible to cross this creek by swimming.]

but, when i saw this sentences below, sometimes i can't understand totally.
i have an idea that i have to correct "to say" as "saying"

[Thank you. It's nice of you to say so.]
(to say->saying)
as you know , The one to be said tells "It's nice of you to say so" after hearing the caller's voice.

'the same reasoning applies to blew sentence

[It's so stupid of you to say so.]

if i wrote this sentence like this.

[ It's so stupid of you saying so.]
is it wrong of Grammer? if yes, why is it wrong and weird?
or What's the difference of the meaning?