Watashi meaning "as for me"

Hello everyone, my question is about the word わたし(watashi). anywhere I learn Japanese they say watashi can mean "as for me" and "I" but a Japanese person I know disagrees about watashi meaning "as for me"

For example here was our conversation:

Me: Nihongo wa muzukashī desu yo

Friend: soudesuka? Muzukashi nai yo!

Me: Watashi wa muzukashii

She told me that I said "I am difficult" but if watashi can also mean "as for me", I mean't to say "as for me, it is difficult" So is she right or am I right?

"Watashi wa," together with the particle, can be used to mean "as for me." But not "watashi" by itself." That is the explanation the textbooks give to say that "...wa" is a topic marker.

But for that conversation, I would have said "watashi ni wa muzukashii," meaning "To me, (the Japanese language) is difficult."

https://youtu.be/fQKy44eVYT4?t=27s

here is an example of my what I learned.

I only watched the sentences on the screen for the first part, but here is what I would teach (former Japanese language teacher here).

- Suki na doubutsu wa nan desu ka? (What is your favorite animal?)
Then the most "natural" answer would be, if I am the only person being asked, meaning, in a one-to-one conversation,
- Inu desu. (It's dogs.)
... because the topic of the sentence, "suki na doubutsu" is known already, you can omit that part.

Only if I, in a group of people, am being asked, and I want to highlight that "Speaking about MYSELF, no-one else," giving a contract from others, would I say:
- Watashi wa inu desu.

For that sentence you had, the proper way to say "The Japanese language is difficult for me" is:
- Nihongo wa watashi ni wa muzukashii desu.
And even though you can omit "Nihongo wa," because the topic of the sentence has been "the Japanese language," you cannot omit "watashi NI wa," the important bit about "TO me (it is difficult)."

I see, thank you very much