What is mistrust?
Far from reality, drawing conclusions by coming under self-imposed pressure of one's own mind. This is the poisonous thought that eats away at the foundations of love like termites.
If you see someone talking softly, you think: 
"They are speaking ill of me!"
If someone takes a little while to reply, you pronounce the verdict:
 "They are ignoring me!"
You greeted someone or started a conversation, and they didn't reply (or didn't hear because they were busy).
"They don't like talking to me anymore, they consider me inferior." (Although they may be immersed in some deep trouble).
Someone didn't invite you to a small gathering or program.
"These people don't consider me to be on their level or they are jealous of me." (Although it is possible that they were limited to close family members only).
You messaged, the other person 'Seen' it but didn't reply immediately.
"She is ignoring me, now I will never reply to her either." (Although she may have left the mobile after getting busy with work).
You entered the room and the two people present smiled at each other.
 "They must be making fun of me or laughing at my clothes/appearance."
You were in trouble and someone apologized for helping at the last minute.
 "They are selfish people, no one helps in times of need." (Without thinking that perhaps their own compulsion is greater than yours).
During the conversation, someone interrupted you or started their own conversation.
"They are not interested in my talk, they consider themselves better than me."
We invent the answers to questions ourselves,
 we decide the culprit ourselves and we pronounce the sentence ourselves.
 In the meaning of the Hadith, you ﷺ gave the best prescription to avoid the poison of this negative thinking:
"Beware of suspicion, for suspicion is the biggest lie." (Sahih Bukhari)

Similarly, Allah Almighty says in the Holy Quran:
يَا أَيُّهَا الَّذِينَ آمَنُوا اجْتَنِبُوا كَثِيرًا مِّنَ الظَّنِّ إِنَّ بَعْضَ الظَّنِّ إِثْمٌ
"
O you who believe! Avoid much suspicion, surely some suspicion is a sin."
We often take things "personally". We feel that every event in the world revolves only around us.
If you are hurt by someone's behavior, instead of setting up a "court" in your mind, have a "benevolent" suspicion of that person, or ask lovingly once. Most misunderstandings are resolved by simply asking for a "clarification".
Our biggest mistake is that we have closed the door of "Husn-e-Zan" (good assumption) and started looking at the world through the "window of doubt". We have become the judges of someone's intentions. However, only Allah knows the state of intentions.
Mistrust not only destroys your peace of mind, but also turns into ashes the relationships that are built with years of hard work. Stop making stories in your mind, talk straight. Give certainty in place of doubt and benevolence in place of mistrust, because peace resides only in a "pure heart".
From the pen of: Za-Sheikh