Comma Placement in a Semicolon-Split Sentence

The addition of the word "knowing" between "panic" and "that" indicates that the following clause (my self-imposed isolation is for naught) is non-restrictive, so a comma is required. This is because - as the sentence is currently worded - the important fact is the panic, not what causes it, and the meaning of the sentence would not be materially changed by excising "knowing that my self-imposed isolation is for naught".

Without a comma, you would need to make the clause restrictive/essential, by either removing the word "knowing", or adding the word "from" before it - thus shifting the importance from the panic to the cause.