I'm reallyve been feeling really sleepy all day long. I've been feeling really sleepy all day long.

"I'm really feeling sleepy" works alone, but to describe when it started + you still feeling this way, you need "I've been"

I'm reallyve been feeling really sleepy all day long. I've been feeling really sleepy all day long.

In addition, because of today’s important works, I was very nervous for at long time. In addition, because of today’s important work, I was very nervous for a long time.

In addition, because of today’s important works, I was very nervous for at long time. In addition, because of today’s important work, I was very nervous for a long time.

"important work" or "important tasks" would work here.

Theseis made me tired. This made me tired.

The reason you say "this" and not "these" is because the hidden word in this sentence is "situation" => This (situation) made me tired. Alternatively, you can say "These things made me tired" but this would be a bit less natural

I definitely think that everyoneabsolutely nobody should NOT wake up early on Monday.s! I definitely think that absolutely nobody should wake up early on Mondays!

Everybody should not = nobody should (more concise and natural) In texts and online, using all-caps for emphasis is pretty normal, but formally, it's not considered correct. Instead, you have to use your words and/or an exclamation point

I definitely think that everyone should NOT wake up early on a Monday. I definitely think that everyone should NOT wake up early on a Monday.

either "on a Monday" or "on Mondays" would work here.