Se tromper should be an easy verb for people to learn since January 20th 2017.
Bonjour. Je m'appelle Donald.
Je me trompe, je me suis trompé et je me trompe toujours.
This is used to acknowledge you were wrong (I was mistaken, I made a mistake, I got it wrong, I was wrong, etc.): "Je me suis trompé, j'ai eu tort, j'ai fait une erreur...".
Fooling oneself is a bit stronger, I think. It means that you pass judgment on yourself (in severe terms) when you realize you have ignored the truth, lacked in common sense or been deceived.
"Je me suis fourvoyé(e), j'ai été bête/idiot(e), je me suis fait des illusions..." would better fit in my opinion.
I'm not sure why we can no longer report other correct solutions, but shouldn't je me suis trompée also be accepted?
Using "se tromper" - "I am wrong" = "Je me trompe [but given Sitesurf's comment, you would probably use "J'ai tort"].
I think the problem you are having is because the French cannot be directly translated into English, and in the English "wrong" is an adjective. Perhaps if you thought of it as: "I misled myself," it would help.
I am just trying to work this out.
"trompé", as an adjective = "deceived". Present tense "Je suis trompé" / "I am deceived"; passé composé "J'ai été trompé" / "I was deceived"
"se tromper" = "to mislead/deceive oneself" = "to be wrong/to make a mistake"
Present: "Je me tromp" = "I mislead myself" = "I am wrong/mistaken"
Passé composé: "Je me suis trompé" = "I misled/have misled myself" = "I was wrong/I was mistaken/I made a mistake."
The lack of distinction does not apply to "se tromper", where the present and the compound past are distinct. "Tromper" is also used non-reflexively to mean "to deceive"
Present pronominal: je me trompe = I deceive myself / I am deceiving myself
Passé composé pronominal: je me suis trompé(e) = I deceived myself / I have deceived myself
Present passive: je suis trompé = I am deceived
Passé composé passive: j'ai été trompé = I was deceived / I have been deceived