我已經學咗啲日文，但我惑到係我覺得廣東話比日文困難.

We say 「但係」 instead of 「但」.

Not sure about that 「惑得」 part. Perhaps it's something from Japanese? We say 「覺得」 when we talk about how we feel: 「我覺得好舒服。」 ( "I feel comfortable." )

Also, if we say something is difficult, we usually just say 「難」 instead of 「困難」. Nothing wrong though.

對唔住，我識知道呢個貼帖子係淺但我唔識許多個係好識廣東話.

「識」 and 「知道」 both translates to "know", but 「識」is used when you have knowledge of something, like「我識英文。」( "I know English。" ). Whereas 「知道」is used more like having a some kind of information, like 「我知道條路好長。」( "I know the road is long." ).

「唔識許多個廣東話」 will mean " I do not know many Cantonese." To say don't know much Cantonese you should say 「唔係好識廣東話」 instead. The phrase 「唔係好...」means something like "Not really... / Don't really...", eg. 「我唔係好想...」: "I don't really want to...", 「我唔係好清楚。」： "I'm not really sure." etc...

Also, in Hong Kong, we usually just say post instead of 「帖子」. That's just something extra.