Mouthful vs. Mouth Full

Mouthful is the amount of food or drink that we put in our mouth at one time.
She took a mouthful of water.
'Mouth full' are two words.
I have my mouth full of food, i.e. my mouth is full of food.

Okay, so I can't write, "I got a mouth full." I have to write, "I got a mouthful," correct?

Usually, mouthful.
"He took a mouthful of water."
However: "She had her mouth full of food, and could not answer.

Neither is correct. "I have", not "I got."

No, "have" is present tense, and I was referring to a past tense situation. For example, "Don't worry. He got a mouthful." Meaning, he got a lot of it in his mouth, and of course swallowed it all.

Mouthful would be correct.

That's a mouth full! Or ... That's a mouthful! (referring to a particularly long job title)

It depends on what sentence you are using. If you are saying someone has a mouth full of food then you would use mouth full. But if you say they just said a big mouthful you would use mouthful. It can be hard to understand but after a little you can get it. It just takes practice to be able to get it.