I've found myself becoming a little bit lazy here recently. I've found myself becoming a little bit lazy here recently.

I have found myself becoming a little bit lazy here recently. I have found myself becoming a little bit lazy here recently.

Not only didhave I writeten less, but I have also corrected fewer Chinese entries. Not only have I written less, but I have also corrected fewer Chinese entries.

Present perfect tense needed here because you are talking about a period of time in the past up until now rather than a situation that only occurred in the past and is completed.

Firstly, Iit's not about the lack of time. Firstly, it's not about the lack of time.

Firstly, Iit's not about the time. Firstly, it's not about the time.

It becomes capitalized only at the beginning of a sentence.

Firstly, Iit's not about the time. First, it's not about the time.

"Firstly" sounds a little formal. Other options: The first thing I realized was... One thing I realized was... First of all...

I've been busy recentlyquite busy these days, but not as busy as last year. I've been quite busy these days, but not as busy as last year.

I swapped out "recently" since you used it a lot in this essay. I added "quite" for emphasis. Here are more options: these days as of late lately

It may be because my English learning has reached a plateau, and I can hardly feel my progress recentlylike I am progressing. It may be because my English learning has reached a plateau, and I hardly feel like I am progressing.

It may be because my English learning has reached a plateau, and I can hardly feel my progress recentlately. It may be because my English learning has reached a plateau, and I can hardly feel my progress lately.

Another word you can use

It may be because my English learning has reached a plateau, and I can hardly feel my progress recentlyanymore. It may be because my English learning has reached a plateau, and I can hardly feel my progress anymore.