Today could've been "Day 37" if I had kept writing daily.

At this time, it's no longer possible to continue the streak, so we need "could've been" as opposed to "could be"

Although there won't be a flame icon next to my name with a big numbers, but I think it's not tha doesn't matters.

Pick one, "although" or "but," not both.
It would always just be one number (however large), e.g., 5000 days in a row is still one number.

I just need to learn to continue my projects after (taking) a break.

And I'm not cheating: - breaks don't count.

A dash or a comma would work better here than for a colon, which is typically going to be used before describing a list.

So it's pretty surecertain that I'll no won't be able to finaccomplish 100 writing days in 100 real days.

You could say "I'm pretty sure" but "it's pretty sure" is unnatural.
The 100 posts in 100 days is a sort of challenge/goal, which are things we accomplish

It's already totally impossible.

The word "impossible" doesn't need a modifier, e.g., totally, very, really. It's already at an extreme.

(But) the only thing that matters is progress.

"But" is adding nothing here and should be omitted

If I'll do my 100 days in 110, then what?

If I've improved a lot by the end of the 110 days, then everything else is irrelevant.

Keeping the hypothetical situation in present tense, "If I do, If I improve..."
Referencing the aforementioned 110 days => referring to "the" days

Of course, I count the days when I write, but only because I like playing with numbers and statistics.

So my lesson for today is "Ttake a break if you need, then keep going!"