If a person talks to himself about someone else who mistreated him, is that regarded as backbiting?
Question

                If I am angry or upset at someone and I lock myself in my room and talk about him behind his back TO MYSELF (not in a gathering or with anyone) then is it still considered backbiting? Will they take from my deeds? Or is backbiting only when talking to others? Please reply to me in English. If this is considered backbiting, how do i repent and expiate it?
Praise be to Allah.
    The basic principle with regard to insulting people and putting them down, 
    or mentioning their faults is that it comes under the heading of bad 
    attitudes and abhorrent behaviour, whether that is done verbally or in one’s 
    heart. It is one of the faults that may have an impact on one’s tongue or 
    heart, and generate in his heart resentment, hatred and scorn towards 
    people, and it distracts a person from the sublime aim for which he was 
    created, which is to fill the world with righteous deeds and good attitudes. 
    The heart that is focused on hating people and reviling them is far removed 
    from purity and far removed from that humility before one’s Lord which is 
    tranquillity and contentment with that which is with Allah, may He be 
    exalted. Allah, may He be glorified and exalted, says (interpretation of the 
    meaning):

    “Verily, those who believe (in the Oneness of Allah - Islamic 
    Monotheism) and do righteous good deeds, and humble themselves (in 
    repentance and obedience) before their Lord, - they will be dwellers of 
    Paradise to dwell therein forever”

    [Hood 11:23].

     Imam al-Ghazaali (may Allah have mercy on him) said, in a chapter on the 
    prohibition on backbiting in one’s heart: 

    Just as it is prohibited for you to speak ill of someone verbally in front 
    of others, it is also not right for you to talk to yourself and think ill of 
    your brother…  The more you have bad thoughts about a Muslim, the more you 
    should care for him and pray for him, because that will annoy the Shaytaan 
    and push him away from you, so that he will not instil bad thoughts in you 
    lest you resort to offering supplication for your brother and caring for 
    him. Whatever faults you find in your brother, offer him advice in private 
    and do not let the Shaytaan deceive you and call you to backbite about him.

    End quote from Ihya’ ‘Uloom ad-Deen (3/150). 

    This applies if you revile and impugn him in secret him with no 
    justification. But if there was some justification and it was done to ward 
    off the mistreatment of a wrongdoer or transgression of a transgressor, then 
    there is no sin in that and no blame. Allah, may He be glorified and 
    exalted, says (interpretation of the meaning):

    “Allah does not like that the evil should be uttered in 
    public except by him who has been wronged. And Allah is Ever All-Hearer, 
    All-Knower”

    [an-Nisa’ 4:148]. 

    However it is also preferable not to revile and impugn him, and to focus on 
    that which will rectify and purify the heart. 
On 
    our website we have previously answered a similar question in detail; please 
    see fatwa no. 136415. 

    And Allah knows best.