“But I still have trouble expressing myself.”

You're studying a 3rd language. Your friend asks how it's going, and you have just told her that you can understand what's going on around you. But you have trouble explaining your thoughts sometimes, so you say:

But I still have trouble expressing myself.

have trouble (doing something)

Another way to say this is "It's hard to (do something)":

If I HAD to pick something, I'd say it's that I have trouble saying "no" to people's requests.

The version "have trouble ___ing" sounds a little bit more formal than "It's hard to ___".

express (oneself)

The word "express" means something similar to "show" or "tell". You can express an idea or a feeling:

She's never expressed a desire to study art before. I don't know why she's suddenly so into it.

I'm having trouble expressing exactly what I want to say.

But, as you can see from the example at top, you can also express yourself. This means to show your personality and what you're thinking.

Of course you don't feel comfortable expressing yourself freely at work. No one does!