Motivational Self-Deception
Motivational Self-Deception is like when somebody flatters you, you believe what they say and are pleased because you want it to be true.
Some say Motivational Self-Deception is really mistaking a desire to believe or motive to belief for a reason to believe. But that would not be self-deception. If you accept good evidence and base belief on it you are not deceiving yourself if the evidence is fake. You may be deceived by somebody but that is not you deceiving yourself. Motivational Self-Deception is really pretending that wanting something to be true makes it believable. You lie to yourself that something that is not evidence is evidence.
We need real reasons to believe. If we are fully rational creatures we would believe what the evidence points to not what we want to believe. We rarely do that especially if we are into religion.
When the emotional reasons for wanting to believe fade away, the person sees the self-deception for what it is.
You will not intentionally deceive yourself unless there is something motivating you. All self-deception consists of intentional self-deception and motivational self-deception.
Intentional Self-deception seems to be the worst. It seems calculating and deliberate. It is the worst in the sense that it is clearer to the practitioner that he is lying to himself than with motivational self-deception. Motivational self-deception shows a bias against truth. Both of them attack integrity. Extremely clever people can believe utter nonsense. This is because they are able to elaborately and cleverly invent reasons to believe and manipulate their perception of the evidence to "believe".

Take the idea of a student who has to spend time trying to make himself see that that the teacher was wrong. The longer the person spends in developing motivational self-deception the more they are using intentional self-deception. The more time you spend twisting things the more you see through what you are doing.
If you believe things because you feel them you can engage in motivational self-deception quickly. Also, you could be using a plan to make yourself believe without being aware of it.
Nobody can lie to themselves without intending to mislead other people too when possible or when the opportunity arises.
When it is Intentional Self-Deception
You can intentionally give in to a motive that causes self-deception. Therefore a lot of what is classed Motivational Self-Deception is really Intentional Self-Deception. Also, when you murder, your motive to commit murder is believed to indicate the presence of an intention to murder. You can have subliminal intentions too. When you hit a man to save your mother you may not even realise you are doing it to save her. You don't have time to spot the intention but it is there.
It should be assumed then that if a person is deceiving herself or himself that it is Intentional Self-Deception that is at work.
No vice or virtue stands alone. To be merciful to an evil person you have to be considerate of the victims of that person, you have to show courage, prudence and wisdom. To lie to yourself you need the vices of egotism, stubbornness and sloth to name a few.
The number of people who are intelligent is rather small. And most of these may be intelligent in one thing and not another. For example, the useless writer may be a genius-level mathematician. The less you know and the less intelligent you are, the more prone you will be to self-deception. And your resources for seeing through it are severely limited.
Most people are irrational. This causes problems for the right of freedom of speech. In principle, we should be allowed to say what we want, even if it is hate speech, as long as the other side can be presented and we can present arguments and the case for our position and the target of our abuse can respond. But too many people will not think rationally about it. Religion does not emphasise reason which means it encourages this problem. It does not help people to reason.