What is more common or more correct (or both of them equals)?
1) It's so much difficult to me to express myself
or
2) It's so much difficult for me to express myself
1) It's so much difficult to me to express myself
"It's so much difficult..."
You appear to be caught between two ideas. On one hand, you were trying to keep it simple but chose the wrong synonym (so much instead of very). On the other hand, you were on the right path, but forgot to insert the adverb more, meaning "to a great extent." So either/or of the following is a good start:
"...to me to express myself."
Again, you appear caught between two ideas. It could be that you simply selected the wrong preposition (to instead of for). Or maybe you forgot to use commas to set off a parenthetic element. The second half can be completed in either of these two ways:
That leaves us with four options to complete this sentence:
I think we can use these formulas:
"...difficult for somebody to do something."
Example: it is difficult for him to learn English at his age.
"... difficult to do something."
Example: It is difficult to learn English when you getting old.
"difficult for me" is correct. The Merriam-Webster dictionary provides usage examples including "These changes will make life difficult for everyone involved."
In short, 2) is your answer.
It's so much difficult for me to express myself.
The pattern is It's + adjective + for someone + to do something
You would find this pattern used almost everywhere.
for example
It is difficult for English learners to know whether to use a gerund or an infinitive after a verb.
source: VOA
Another reference is Ngram Viewer, clicking the links pointed out by the red arrow, one would see the same pattern.
In sentence 1, 'It' is a pronoun, and you need to say ‘Expressing myself is difficult to me.’ instead of ‘It's difficult to me to express myself.’ Sentence 2 is grammatically wrong.
Sentence 2 is correct because 'it' here is a subject and means 'to express myself'. So you can say 'For me to express myself is difficult' instead of the original one.