* "Just" and "only" have very similar meanings in this context. They both emphasize that something is the sole or primary thing.
* Using both words together is unnecessary and makes the sentence sound awkward.
You can correct the sentence by choosing one of the words:
* "I am just..." (emphasizing the simplicity or limitation of the statement)
* "I am only..." (emphasizing exclusivity)
The specific choice depends on the intended meaning of the sentence.