How to say "self-fulfilling prophecy" in Mandarin/Cantonese?
self-fulfilling = 自证

Here, self = 自. You might see other place translate self to 自己。 That's because, in ancient Chinese, every word has only one character. In current Chinese, most words contain more than one character, such as 自+己 -> 自己。 But when in words that format as combinations, every word only use their original character. That is 自 here.

Fulfilling has no exact translation in chinese. So 证, which means proven, is used here.

Here is something more. 

"If you remain this negative, your thoughts will become a self-fulfilling prophecy." 

You can translate it word by word：如果你继续这么悲观，你的思想会陷入自证预言.

If this is a conversation between native speakers, a more common way is using idioms, something like 智子疑邻 or 杯弓蛇影。