“I feel like I've hit a plateau.”

You're learning to play the guitar. For a while, you were getting better and better, but now you feel like your skill level isn't improving. You tell your friend who plays guitar:

I feel like I've hit a plateau.

I feel like (clause)

You use "I feel like ___" to express your opinions. When you state your opinions this way, it sounds more personal than when you say "I think ___". You can also use "I feel like ___" when you're not quite sure of something. For example, if you lost your credit card:

I feel like I might have left it at the bar I went to the other night.

I've (done something)

You use "I've ___" to talk about something that recently happened to you, and hasn't changed yet. In the example above, "I've hit a plateau" means that you first hit a plateau at some point recently and still aren't improving.

hit a plateau

A "plateau" originally means a kind of mountain with a flat top:

But there's another meaning of "plateau" that comes from its shape. On a graph, if you show something growing quickly and then staying the same for a while, it looks like the shape of a plateau.

So, to "hit a plateau" means to stop growing or improving at something. You usually use this phrase in speaking or in writing, to talk about things you're learning.