Once I believed ithis strongly until Ibefore graduateding from university and entereding the professional world.

After too many unfair and cruel competitions, I thought I would never believe in this sentence again, but recently somethingmy mindset has changed.

It turnsed out to be a series of courses to instruct those [UNCLEAR: elder sisters from 0 to 1].

NOTE: I could not understand the phrase "elder sisters from 0 to 1." Could you explain a bit more? Do you mean women aged 100 or 101?

What's more, the age range of theose yoga beginners iwas roughly 50 to 70 (years old).

Those older sistpractioners are the most serious and understanding learners.

They not only gave me a sense of accomplishment, but also made me start to think about what I want to do in retirement when I get old - I've never taken thishought about things seriously before.

Once I believed it strongly until I graduated from university and entered the professional world.

After too manyuch unfair and cruel competitions, I thought I would never believe in this sentenceaying again, but recently something has changed.

It turns out to be a series of courlasses to instruct thoese elder sisters from 0 toladies from the very beginning to level 1.

A course is a series of classes.

“Those” is OK, but it puts some emotional distance between you and them. “These” makes you sound closer to—or more friendly toward—them.

“Ladies” is a polite equivalent to “姐姐们” in this context. We don’t use “elder sisters” in this way.

“From 0 to 1” might work in some contexts, but here it requires more explanation.

After the first courselass last weekend, I found this special yoga teaching has given something good back to me.

Those older sisterladies are the most serious and [understanding] learners.

Here, I take “understanding” to mean “forgiving of my mistakes or shortcomings.” Is this what you mean? Or do you mean something like they understand your instructions quickly?

When yYou dDo sSomething gGood

In titles, all words are normally capitalized in English

Once II strongly believed it strongly until I graduated from university and entered the professional world.

After too many unfair and cruel (competitions), I thought I would never believe in this sentence again, b. But recently, something has changed.

I don't think competitions is the correct word here. A competition is usually a sporting event or other event where there is a clear winner. This rarely happens in the work environment.

I thought it was a charity event to celebrate Women's Day, so I said yes without any hesitation.

"Any" is not needed here

It turnsed out to be a series of courses to instruct those elder sisters from 0 to 1.

Since your previous sentences were past tense, I made this one in the past as well. It's not clear what you mean by "from 0 to 1" and I'm afraid it makes no sense to me here. If you can explain what you mean, I'll give you the correct phrasing in English.

Last week, I needed to spend almost all of my spare time studying the physiology of older women and rearranging the content based on themy studyies.

And now, my great-grandmother's words cahave come back to me.

They not only gave me a sense of accomplishment, but also made me start to think about what I want to do in retirement when I get old - I've never taken this thingat seriously before.