sorry probably a stupid question. but what is the ce doing there?
When "what" is not an interrogative word, you can translate it to "ce qui" if it is a subject or "ce que" if it is a direct object:
it is this that i told him
Why is this wrong?
Use natural English when making your translation. The literal, word-for-word method really doesn't work here. Perhaps you understand the meaning--now put it in standard English.
I wrote "It is that which I have told him" and got it wrong. Perhaps it's because my profession requires me to use lengthy, wordy, stilted language that I didn't translate my English into "natural" English, lol.