I'm in a disbelief.

It seems like you're expressing surprise or shock, but the phrase is slightly off. The correct expression is:

"I'm in disbelief."
This means you're having trouble believing something or are shocked by it.

For example:

"I’m in disbelief that we won the lottery!"

"She was in disbelief when she heard the news."


Let me know if you want more examples or clarification!