Do you use a semicolon or colon before the word “that” in a sentence?

I suspect it’s a question of style, however the word ‘that’ in this situation replaces the expository colon.

I want to tell him: maybe it is the fact that I have been

I find the sentence:

• I want to tell him that maybe it is the fact that I have been guarded every second of every minute of every hour of every day that I have been here; that I cannot even relieve myself when nature calls without my guards knowing.

faulty from the point of view of grammar and punctuation.

The clause "that I cannot even relieve myself when nature calls without my guards knowing" is the main clause in a cleft sentence. What wants to be emphasized by means of the cleft structure is the reason why the person cannot feel free even when nature calls, and this is why the sentence is defective: there should be a conjunction or preposition of reason before "the fact that":

• I want to tell him that maybe it is because of / due to the fact that I have been guarded every second of every minute of every hour of every day that I have been here that I cannot even relieve myself when nature calls without my guards knowing.

or

• I want to tell him that maybe it is because I have been guarded every second of every minute of every hour of every day that I have been here that I cannot even relieve myself when nature calls without my guards knowing.

The semicolon before the main clause is definitely wrong. There could be a comma, but not a semicolon or a colon. We should consider that the sentence stems from:

• I want to tell him that I cannot even relieve myself when nature calls without my guards knowing because I have been guarded every second of every minute of every hour of every day that I have been here => I want to tell him that it is for this reason that I cannot relieve myself.

I read the sentence as follows:

I want to tell him that maybe it is the fact

1. that I have been guarded every second of every minute of every hour of every day that I have been here; [and]
2. that I cannot even relieve myself when nature calls without my guards knowing.

In other words, we simply have a list of two that-clauses that are connected by a conjunction, except that the conjunction itself has been omitted for effect.

I think the reason that the two items in the list are separated by a semicolon is that they are quite long. I believe this is one of the commonly accepted uses of a semicolon.

A semicolon would certainly be inappropriate if there were only one that-clause rather than a list of them. You definitely wouldn't write "I want to tell him that it is the fact; that I have been guarded every second I have been here."