One of the important teachings of Islam is to be truthful. It is no doubt

relevant when dealing with people/society.
However does it apply equally in the realm of our psychology & subsconscious
mind? In Psychology,it is a well known fact that one can change oneself thru
positive self-affirmation & visualisation .It is believed that the
self-affirmations to the subconscious mind is best done in an “already achieved
mode”. For example,a student who is weak in mathematics and wish to improve on
it should say the self-affirmations as “I am excellent at mathematics” & also
visualize himself as ALREADY being good at Maths repeatedly. Or a person who is
a coward could reprogram his mind by saying to himself,” I am brave”,etc.This is
so that the subconscious mind is set with the positive intended goal and thus
will program the whole body with the message.
But the question is that such self-affirmation is not the truth (or not the
truth yet).
When he is telling himself that he is ALREADY being good at Maths, he is
actually “lying “ to himself. Is it acceptable in Islam?
In surah al-anfaal verse 43 & 44 describes how Allah makes the Muslims see the
enemy as small in number whereas they are actually LARGER than the Muslims. So
it seems it is allowable to “lie” to oneself in a positive way?
Also the concept of “husnul zann” of giving “70 excuses” when dealing with
Muslims show that we prefer to see the positive aspects rather than the real
truth?

I am so reminded of 76 Trombones in the musical "The Music Man", with Robert Preston and Shirley Jones. The "thinking method" of learning music didn't seem to work then.

Facts: Your statement "In Psychology, it is a well known fact...", one must remember that facts can be true or false, and most "facts" are not true.

Now your question: Lying is lying, the famous "white" lie, is still a lie. Is is acceptable in Islam? Is lying acceptable?

Now enemies of Islam: I am wondering whose perception contains these enemies? If Islam lists as "enemies" as all who do not agree, or believe, it is Islam that is doing that, not the others. Richard Nixon while President of the United States kept a secret "Enemies List": those who critized or did not agree with him. Many of them were surprised that when disclosed, they were on the Enemies List, it was in Nixon's mind, not theirs. So when you state the Enemies of Muslims are greater then Muslims, exactly who is deciding who is an "enemy"?
I don't know about husnul zann and making excuses. I deal with folks making excuses each day, I have grown accustom to it, and expect it as a normal response. I don't see that avoidance technique as a Muslim issue. Yes, the dog does eat homework, but it is a rare event.