Trojan horse means, something that appears safe from the outside, but sabotages from inside. “Trojan horse” word came into existence from the popular Trojan war, in which Greeks entered into the Trojan kingdom hidden in a great wooden horse and destroyed the entire kingdom.
In real life, if you have a psychological disorder, normally you recognize it and try to come out of it. But this is not the case always. here is a trojan which brings us pleasure, yet tries to destroy us by keeping you depressed always. That is self pity.