After using it a few times, I lost fresh feelings about iit became old news fast and I stopped using it. After using it a few times, it became old news fast and I stopped using it.

"I lost fresh feelings about it", sounds quite poetic and not the correct register for this piece of writing. I gave an example of what you could write instead.

The main reason, I think, maybe that is that maybe its lid was too clumsyawkward and I was not used to it. The main reason, I think is that maybe its lid was too awkward and I was not used to it.

Wouldn't use clumsy, here maybe - fiddly, awkward

Because it was winter back then, I felt I should buy an insulation cup so that the tea water wouldn't be cold verycool down as quickly. Because it was winter back then, I felt I should buy an insulation cup so that the tea water wouldn't cool down as quickly.

You could say - wouldn't cool down as quickly or heat wouldn't escape as fast

I checked insulated cups on the internet to find out which brand had betterthe best quality and finally, I made athe decision to buy a Japanese brand "zZojirush". I checked insulated cups on the internet to find out which brand had the best quality and finally, I made the decision to buy a Japanese brand "Zojirush".

It was a 360 ml volume cup and had really had a good quality of keeping warmgreat heat insulation. It was a 360 ml cup and had really great heat insulation.

I would remove 'volume' here, it's unneeded

But it didn't have a filter screen, so often times, the tea leaves would enter my mouth when I drank tea. But it didn't have a filter, so oftentimes, the tea leaves would enter my mouth when I drank tea.

I felt I should buy another teacup with a filter screen, thus it can not only can prevent tea leaves from entering my mouth, but also be very useful for me to wash tea leaves before I make tea. I felt I should buy another teacup with a filter, thus it can not only prevent tea leaves from entering my mouth but also be very useful for me to wash tea leaves before I make tea.

I still wanted to buy the "zZojirush" brand because its keeping- warm functionheat insulation feature was very effective. I still wanted to buy the "Zojirush" brand because its heat insulation feature was very effective.

When talking about a company or brand, capitalise the first letter - amazon / Amazon - subway / Subway etc When talking about keeping something like tea warm in a cup, you would use a word such as insulation or heat insulation