LThe last two weeks I focused on exercises, and I am happy with the results I got. The last two weeks I focused on exercises, and I am happy with the results I got.

Also, my knowledge on how to exercises is getting better. Also, my knowledge on how to exercise is getting better.

Or "how to do exercises" - "to exercise" is a verb, "to do exercises" is verb "to do" plus noun "exercises".

TI don't want to get too tired this week, because I'm working inon the night shift. I don't want to get too tired this week, because I'm working on the night shift.

This sentence fragment doesn't say anything on its own, so I joined them and rearranged.

So, I decided to mainly stay in the library and read some books. So, I decided to mainly stay in the library and read some books.

Same experienceThe same thing happened! The same thing happened!

Or "I had the same experience!" I don't think we talk about experiences "happening".

After the first day I just want to quit! After the first day I just want to quit!

Maybe "I just wanted"? But "want" is okay if it's still hard, if you still have trouble keeping the routine.

But somehow, I manage to keep continuedoing this task. But somehow, I manage to keep doing this task.

Again, "managed" if you're talking about specifically what happened yesterday but "I manage" is okay if you want to say this happens every night, that it's a habit now. "I manage to keep doing" or "I manage to continue". Don't say "keep continue".

Starting from tomorrow I'm going back to the exercise cycle ver, and I'm really looking forward to it. Starting from tomorrow I'm going back to the exercise cycle, and I'm really looking forward to it.

This sentence needed a lot of things!