kind of (adjective/adverb)

"Kind of" means "a little" or "somewhat". People often use it in spoken English:

I'm kind of shy when it comes to the opposite sex.

It kind of took me by surprise.

You can use "kind of" before an adjective ("kind of shy") or before a verb ("kind of took me by surprise").

Another phrase with a similar meaning is "somewhat":

Most students find that university courses are somewhat more difficult than the classes that they took in high school.

"Somewhat" is more formal. Use "kind of" for most situations and "somewhat" when discussing academic topics or in writing.

The pronunciation of "kind of" sounds like "kinda".

I've been (adjective) lately

This is a way to describe how you've acted for a few days or months that's different from how you were in the past. When you describe yourself in this way, it sounds like this is a characteristic that isn't usual for you.

You can use a simple adjective like "tired":

I've been pretty tired lately.

Or you can use a longer phrase like "in my own little world" in this example:

Sorry, I've kind of been in my own little world lately.

The phrase you use should be similar to an adjective, meaning that it should describe the noun ("I" in this case).

There's a little confusion about the difference between "recently" and "lately". "Lately" is more appropriate for continuing situations, while "recently" is better for events that happened just once, a couple of times, or that have continued but are finished now:

Have you been going to the gym a lot lately? You look great!

I ran into Stacy recently at the Food Emporium.

be in (one's) own world

This means to be focused only on your own thoughts, and to not pay attention to other people or events. This phrase is a metaphor. It's comparing your thoughts to being on a planet with no other people.

You can add "little" to this phrase to make the thoughts you're having "in your own world" sound unimportant and selfish.