I think this quote of Seneca is very true. Our brain most of the times overfires things and make you feel worried, question things and paranoid sometimes. But this is also true that very few are able to control their thoughts as they understand by heart it's not facts. Do you also think like this? Sometimes most of the suffering is caused through your minds? Maybe the threat isn't that big deal? It's fine when sometimes you can't complete your project on time, it's fine sometimes you reach office late, it's fine to think what you'll do if you had a breakup ( sorry for this). You cannot stop your overthinking but can control the most of it by staying calm and positive.

Mark Twain also said "I am an old man and have lived many misfortunes, most of which has never happened to me." lol