Betrayal..Is it worth the forgiveness?

Recently, my boyfriend and I have been reaching some rough patches. Besides my best friend Shannon F., I try to keep that aspect of my life very personal. I will openly share basic information, but I do not go into vigorous detail with my other close friends about the dynamics and details of my fighting and making up with my boyfriend. Last Friday, I suffered a dramatic and upsetting break-up and like most girls would, I reached out to my close girl friends and let them help me stand up on my own two feet. One of those girls has always shown a sense of bitterness to myself and her best friend having boyfriends, being that she was the odd one out and did not have a boyfriend. That is not my problem, I understand her sense of bitterness because I too have been there. I had been with my boyfriend for nine months and I can remember when the tables were turned and it was me who was single. At our break-up, my friend and I had been arguing the week before. I've been stressed from sports, work, and school and it's been a rough month- she seemed to understand that and was worried about me. However, she has always had a somewhat relationship with my boyfriend that did not include me- a friendship that I was fine with but at times it did seem odd... To get to the point, my other good friend sits with my boyfriend and this girl at lunch and she overheard that girl talking about me to my boyfriend- saying I'm a bad girlfriend and I shouldn't be hysterical crying in school over him and getting upset with him. Again, I keep my life personal from her and she has no idea the reasons why I cried or was upset- it's no one's business but me and my boyfriend. I found this out, felt betrayed, and confronted her, angrily. I had had it up to here with her. She was speechless and tripped over her words but I ended up apologizing saying I should have talked to her first- she said I didn't know everything she said and it wasn't that bad, so I chose to believe her. My boyfriend then confronted me and said, after we resolved things and got back together, that this girl had said I always flirt with this other boy and I am all over other boys. Besides the blatant lies of this, that is complete betrayal. She did not even deny saying this or making it up. She just kept saying, "Whatever. Goodbye." I have stuck by her as she has lost so many people over her attitude and betrayal this past year. Is this forgivable or is it completely going against the code of Best Friends? Her attempt to sabotage my relationship after I apologized to her for yelling at her did not work yet I can still not sleep over it. I feel so dumb for trusting someone who turned on me. Is she worth the forgiving? I know I have friends who will make up for her absense but I feel terrible dropping someone. Help! I'm tossing and turning!