Being betrayed by a close friend is one of the hardest things to go away from your thoughts. Depending on the way you process things that you go through, you can either rise up from it quickly or become damaged and untrusting of everyone to an extent. For me, I had to write about it, go out to the movies, go out with friends more often and sometimes just cry about it. Many times questions would arise trying to wrap your mind around it all, to fully understand why someone so close to you would be deceitful and be normal about it. Then again, you seem to be a nag when you want clarification from them, so most times many questions go unanswered.
However, it is of great importance to have all questions cleared if you live with the betrayer, as sometimes they too may have been confused and is grieving the same way as you are. The issue of trusting them again can only come about after years of learning to trust that same person again or never. In the case of trusting again, for me personally, it has been going on five years after being betrayed be a loved one to see some glimmer of trusting again. I sometimes have to force good thoughts, to abolish the bad thoughts out of my mind! Truly, we all heal at a different rate.