What is the best way to forget bad choices in the past? We all have done bad karma,so if something is bugging my mind constantly 

(Like killing some mosquito)

what is the solution for me,what can i do to overcome it/or to forget it?
Short answer is, you should not forget bad choices. You should learn from bad choices.

Long answer:


  Kukucca is characterized by regret. It's function is to grieve about what has or what has not been done. It manifests as remorse. It should be regarded as
  slavery.


--from Visuddhimagga XIV, 174


  The Dalai Lama was working with an American psychiatrist who was interviewing him for a book on happiness. The subject of remorse was broached: His Holiness explained that one time an elderly Buddhist man came to see him to ask for instructions on how to do a very difficult Yoga pose. The Dalai Lama told the man that he was too old and should not attempt the pose as it would be too dangerous. The old man thanked the Dalai Lama, went home and killed himself so he could be reincarnated as a younger, healthier man who could attempt the pose. After hearing the news, the Dalai lama was overcome with guilt at being the reason for another man’s death.  
  
  “So how did you deal with that?” asked the interviewer. “How did you get rid of the remorse.”  
  
  The Dalai Lama sat there in silence for a minute or two, thinking hard about the question.  
  
  “I didn’t get rid of it” the Dalai Lama explained. “It’s still with me every day. I just continue to live with my heart open.”  


-- from http://toryshulman.com/how-to-live-with-remorse-efficiently

So, what does it mean, "to live with one's heart open"? It means to see it like so: "What was done in the past cannot be changed. Thinking about it will not help me undo it. Now I have learned my lesson and will not make exactly the same mistake. I don't know what will happen in the future, but if similar situation comes up, or even a completely different situation -- I will do my best!"