Right word for repeating something several times to make one believe that the thing is true [closed]

"proof by assertion", a fallacy, must be the phrase you are looking for.

Proof by assertion, sometimes informally referred to as proof by repeated assertion, is an informal fallacy in which a proposition is repeatedly restated regardless of contradiction. Sometimes, this may be repeated until challenges dry up, at which point it is asserted as fact due to its not being contradicted (argumentum ad nauseam). In other cases, its repetition may be cited as evidence of its truth, in a variant of the appeal to authority or appeal to belief fallacies.

This fallacy is often used by politicians during election times.

Stylistically, there are many:

Here they are, some are Latin or Greek-based words, which is OK, since we seem to be discussing rhetoric. Many though are also used in their English form, which I am not going to work through here:

Repetition of ideas commoratio: Dwelling on or returning to one's strongest argument.

disjunctio: A similar idea is expressed with different verbs in successive clauses.

epanodos: Repeating the main terms of an argument in the course of presenting it.

epimone: Persistent repetition of the same plea in much the same words.

exergasia: Augmentation by repeating the same thought in many figures.

expolitio: Repetition of the same idea, changing either its words, its delivery, or the general treatment it is given.

homiologia: Tedious and inane repetition. Unvaried style.

hypozeuxis: an expression or sentence where every clause has its own independent subject and predicate.

palilogia: Repetition in order to increase general fullness or to communicate passion.

pleonasmus: Use of more words than is necessary semantically. Rhetorical repetition that is grammatically superfluous.

scesis onomaton: A series of successive, synonymous expressions.

synonymia: The use of several synonyms together to amplify or explain a given subject or term. A kind of repetition that adds force.

tautologia: The repetition of the same idea in different words, but (often) in a way that is wearisome or unnecessary.

traductio: Repeating the same word variously throughout a sentence or thought. http://humanities.byu.edu/rhetoric/Figures/Groupings/of%20Repetition.htm

I'm not sure whether you mean repeating it to oneself or to someone else, but in case the latter, then:

Propaganda

From the Wikipedia article "Propaganda techniques":

This uses tireless repetition of an idea. An idea, especially a simple slogan, that is repeated enough times, may begin to be taken as the truth.

^{Emphasis mine.}

Repetition is essential to propaganda:

But the most brilliant propagandist technique will yield no success unless one fundamental principle is borne in mind constantly and with unflagging attention. It must confine itself to a few points and repeat them over and over. Here, as so often in this world, persistence is the first and most important requirement for success.

^{Mein Kampf, Adolf Hitler, 1925. Translated by Michael Ford, 2009. Emphasis mine.}

Simply, the term argument by repetition can be found at Logically Fallacious which details over 300 logical fallacies.

I think autosuggestion is close to what you are looking for:

• an influencing of one's own attitudes, behavior, or physical condition by mental processes other than conscious thought.

(M-W)