안녕~^^

오늘은 내가 한국어 많이 공부해~공부만 해~

→ 오늘 난 한국어를 많이 공부했어. 하루종일 공부만 했어~(meaning you spent all your day studying)
지쳤어 ㅠ_ㅠ 그렇지만, 내가 행복해~^_^

→ 너무 지쳤지만, 그래도 행복해.
한국어 아직 잘 못해..할 수 없다.

→ 아직 한국어를 잘 못하지만, 어쩔 수 없어. (I'm not good at it yet, but there's nothing I can do)

ㅎ..ㅇㅋ bye~^^

*some of the sentence wasn't clear, so I had to make some adjustments to it. Hope it's fine. Also to help your understanding, I didn't change the whole sentence but just some parts. Meaning it could sound awkward when you actually say it, but is grammatically not wrong.