I was dumped by my girlfriend after three and a half years. Two days later she slept with another guy.

When we started dating, she found text messages from my ex and emails. They were old but her point was that I shouldn't have kept talking to her. She claimed I cheated by texting...weather if that's true I can say I did it when she told me not to...so hanging over our head has been her claiming she can't trust me, yet we were happy. I did everything for her, supported her in every way. For the past couple of months things couldn't have gotten better. We talked about getting out own place, marrying...suddenly one night we were watching TV she asked if I still talked to my ex- I replied "where is that coming from" she said that she has been feelng like something is going on and she's usually right about that...she demanded to see my EMAILS and PHONE. I didn't show it to her. I argued because I've done this many times in the past and yet a week, a month and it's still the same. I love her very much. She claimed she loves/loved me. Now that night she said if i walked out, it's over. I did...but trying to prove a point...this was thursday...friday she went out with friends, sunday with a guy and she slept with him. She told me that she was upset and it just happened. She told me I shouldn't be upset because we were broken up when it happened. I am devastated. I think partly because all our years together took only two days for her to stray...Is this really what she said it is. She wants a break because she claims she needed to be single and she hates that I was the only one she depended on. She said that she needs something different because knowing her she would come right back to me. She also said if she came back she knows that I would use what she did against her. Tell me how can she want a break to be single and dating already??? where's the healing time? she told me that she has feelings for me and that she wants to be friends but I'm devastated because she meant the world to me. She told me everyday how much she loved me, how lucky she was to have me in her life...yet this. She claimed that she just couldn't trust me after what she found three years ago and that night when she asked me to show the email/cellphone...it was clear. She said she can't keep investing her feelings and getting hurt...what should I do, i want to give her space and I truly wish her happiness...but I know we were happy...i'm running into walls trying to figure this...it's only been a week and I'm in hell...i've even isolated myself making her the only person...stepped back from friendships because she was so jealous...NOW THIS>>>HELP ANY ADVICE...is there a PILL for this???
She really is my best friend, as she claims I am hers, even now after all this. I'm willing to put this behind us and move on...any insight into her actions? Is it really what she claims...?